You should be able to add the armor. Just have a mercenary summoned and click the button in the top left of the merc character window. It will allow you to add armor and such. Looks like a pruple man all spread out.It is the same method and graphic as the default UI.
"Build a man a fire, and he'll be warm for a day. Set a man on fire, and he'll be warm for the rest of his life."-Terry Pratchett