Hi Knurk,
You have to adjust a few CommandButtons in CommandButton.inc and CommandSets in CommandSet.inc (path: data\ini\includes).
If you replace the current CommandButtons and CommandSets with those from below, the archers in the Mirkwood Fortress will be available once you have built the Vault of The King and researched Silverthorn Arrows and Longbows.
If you want to make the archers directly available, you can simply remove NEED_UPGRADE from the Options line and remove the NeededUpgrade line completely.
If you prefer to unlock them through the troop chambers you have to use the upgrade Upgrade_RaiseSchmiedekunst in NeededUpgrade instead of Upgrade_MountAI.
CommandButtons
CommandButton Command_ConstructElvenMirkwoodArcherHorde
    Command                 = UNIT_BUILD
    Object                  = ElvenMirkwoodArcherHorde
    TextLabel               = CONTROLBAR:ConstructElvenMirkwoodArcherHordeMod
    ButtonImage             = BEElvenBarracks_MirkwoodArcher
    ButtonBorderType        = BUILD 
    DescriptLabel           = CONTROLBAR:ToolTipBuildElvenMirkwoodArcherHordeMod
    Radial                  = Yes
    InPalantir              = Yes
    IsClickable             = Yes
    Options                 = CANCELABLE NEED_UPGRADE
    ShowProductionCount     = Yes
    NeededUpgrade           = Upgrade_MountAI Upgrade_TechnologyLothlorienLongBows Upgrade_TechnologyElvenSilverthornArrows
    DisableOnModelCondition = RUBBLE POST_RUBBLE
End
CommandButton Command_SelectRevivablesMirkwoodCampKeepHeroes
    Command                 = PUSH_VISIBLE_COMMAND_RANGE
    TextLabel               = CONTROLBAR:SelectRevivablesMirkwoodCampKeepHeroes
    ButtonImage             = UCCommon_GoodHeroes
    ButtonBorderType        = SYSTEM 
    DescriptLabel           = CONTROLBAR:ToolTipSelectRevivablesMirkwoodCampKeepHeroes
    Radial                  = Yes
    CommandRangeStart       = 10
    CommandRangeCount       = 13
    Options		    = HIDE_WHILE_DISABLED 
    DisableOnModelCondition = RUBBLE POST_RUBBLE
End
CommandButton Command_SelectRevivablesMirkwoodCampKeepHeroes_MapMirkwood
    Command                 = PUSH_VISIBLE_COMMAND_RANGE
    TextLabel               = CONTROLBAR:SelectRevivablesMirkwoodCampKeepHeroes
    ButtonImage             = UCCommon_GoodHeroes
    ButtonBorderType        = SYSTEM 
    DescriptLabel           = CONTROLBAR:ToolTipSelectRevivablesMirkwoodCampKeepHeroes
    Radial                  = Yes
    CommandRangeStart       = 10
    CommandRangeCount       = 14
    Options		    = HIDE_WHILE_DISABLED 
    DisableOnModelCondition = RUBBLE POST_RUBBLE
End
CommandSets:
CommandSet MirkwoodCampKeepCommandSet
    InitialVisible = 10
    1      = Command_ConstructLothlorienMirkwoodFighterHorde
    2      = Command_ConstructLothlorienMirkwoodArcherHorde
    3      = Command_ConstructLothlorienMirkwoodSpearmenHorde
    4      = Command_ConstructLothlorienMirkwoodElkriderHorde
    5      = Command_ConstructLothlorienMirkwoodPalaceGuardHorde
    6      = Command_ConstructElvenMirkwoodArcherHorde
    7      = Command_SpecialAbilityMobilisierungsbefehl
    8      = Command_Sell
    9      = Command_SelectRevivablesMirkwoodCampKeepHeroes
    10    = Command_StartCitadelSelfRepair
    11     = Command_InfoButtonMirkwoodHeroes
    12     = Command_FakeRingHeroReviveSlot
    13     = Command_FakeCreateAHeroReviveSlot
    14     = Command_FakeHeroReviveSlot1
    15     = Command_FakeHeroReviveSlot2
    16     = Command_FakeHeroReviveSlot3
    17     = Command_FakeHeroReviveSlot4
    18     = Command_FakeHeroReviveSlot5
    19     = Command_FakeHeroReviveSlot6
    20     = Command_FakeHeroReviveSlot7
    21     = Command_FakeHeroReviveSlot8
    22     = Command_FakeHeroReviveSlot9
    23     = Command_RadialBack
End
CommandSet MirkwoodCampKeepCommandSet_GdK
    InitialVisible = 10
    1      = Command_ConstructLothlorienMirkwoodFighterHorde
    2      = Command_ConstructLothlorienMirkwoodArcherHorde
    3      = Command_ConstructLothlorienMirkwoodSpearmenHorde
    4      = Command_ConstructLothlorienMirkwoodElkriderHorde
    5      = Command_ConstructLothlorienMirkwoodPalaceGuardHorde
    6      = Command_ConstructElvenMirkwoodArcherHorde
    7      = Command_SpecialAbilityMobilisierungsbefehl
    8      = Command_Sell
    9      = Command_SelectRevivablesMirkwoodCampKeepHeroes
    10    = Command_StartCitadelSelfRepair
    ;11     = Command_InfoButtonMirkwoodHeroes
    12     = Command_FakeRingHeroReviveSlot
    13     = Command_FakeCreateAHeroReviveSlot
    14     = Command_FakeHeroReviveSlot1
    15     = Command_FakeHeroReviveSlot2
    16     = Command_FakeHeroReviveSlot3
    17     = Command_FakeHeroReviveSlot4
    18     = Command_FakeHeroReviveSlot5
    19     = Command_FakeHeroReviveSlot6
    20     = Command_GenericReviveSlot7
    21     = Command_GenericReviveSlot8
    22     = Command_FakeHeroReviveSlot9
    23     = Command_RadialBack
End
CommandSet MirkwoodCampKeepCommandSet_ForAI
    InitialVisible = 10
    1      = Command_ConstructLothlorienMirkwoodFighterHorde_ForAI
    2      = Command_ConstructLothlorienMirkwoodArcherHorde_ForAI
    3      = Command_ConstructLothlorienMirkwoodSpearmenHorde_ForAI
    4      = Command_ConstructLothlorienMirkwoodElkriderHorde_ForAI
    5      = Command_ConstructLothlorienMirkwoodPalaceGuardHorde_ForAI
    6      = Command_ConstructElvenMirkwoodArcherHorde
    7      = Command_SpecialAbilityMobilisierungsbefehl
    8      = Command_Sell
    9      = Command_SelectRevivablesMirkwoodCampKeepHeroes
    10     = Command_StartCitadelSelfRepair
    ;11     = Command_InfoButtonMirkwoodHeroes
    12     = Command_FakeRingHeroReviveSlot
    13     = Command_FakeCreateAHeroReviveSlot
    14     = Command_FakeHeroReviveSlot1
    15     = Command_FakeHeroReviveSlot2
    16     = Command_FakeHeroReviveSlot3
    17     = Command_FakeHeroReviveSlot4
    18     = Command_FakeHeroReviveSlot5
    19     = Command_FakeHeroReviveSlot6
    20     = Command_FakeHeroReviveSlot7
    21     = Command_FakeHeroReviveSlot8
    22     = Command_FakeHeroReviveSlot9
    23     = Command_RadialBack
End
CommandSet MirkwoodCampKeepCommandSet_MapMirkwood
    InitialVisible = 10
    1      = Command_ConstructLothlorienMirkwoodFighterHorde
    2      = Command_ConstructLothlorienMirkwoodArcherHorde
    3      = Command_ConstructLothlorienMirkwoodSpearmenHorde
    4      = Command_ConstructLothlorienMirkwoodElkriderHorde
    5      = Command_ConstructLothlorienMirkwoodPalaceGuardHorde
    6      = Command_ConstructElvenMirkwoodArcherHorde
    7      = Command_SpecialAbilityMobilisierungsbefehl
    8      = Command_Sell
    9      = Command_SelectRevivablesMirkwoodCampKeepHeroes_MapMirkwood
    10     = Command_StartCitadelSelfRepair
    ;11     = Command_InfoButtonMirkwoodHeroes
    12     = Command_FakeRingHeroReviveSlot
    13     = Command_FakeCreateAHeroReviveSlot
    14     = Command_FakeHeroReviveSlot1
    15     = Command_FakeHeroReviveSlot2
    16     = Command_FakeHeroReviveSlot3
    17     = Command_FakeHeroReviveSlot4
    18     = Command_FakeHeroReviveSlot5
    19     = Command_FakeHeroReviveSlot6
    20     = Command_GenericReviveSlot7
    21     = Command_GenericReviveSlot8
    22     = Command_GenericReviveSlot9
    23     = Command_FakeHeroReviveSlot10
    24     = Command_RadialBack
End