I could suggest to create a outpost fortress for Angmar: the Troll Keep, a small fortress of steel over stone from where you can recruit the Snow Trolls and and Hill Trolls, Rogash included. From Rise of the Witch King, I though that the trolls were a main part of the Angmar army, being the only 'chivalry' after the Wolf Riders, and they deserves more that be only a temporal summon.
In the building, you can recruit the Trolls by high prices, costing 90 CP or more by group, but for maintain balance, you can only recruit 3 Snow Trolls groups and 3 Hill Trolls groups. Rogash can be recruited as a hero here.
About the summon power, it can be changed for a passive upgrade that regenerate HP from beasts units (wolves, wolves riders, werewolves and trolls) when they're not fighting, or summon the giant frozen werewolf that Angmar had in the original Rise of the Witch King.
Edited suggestion: changed outpost name and details