I too am on Ascendant and using RGMercs and what i'm doing (which probably is a very sucky way of doing it) is i clicked the custom button on the class tab, and going into the insertclasshere.lua file and copying something that works similar and altering it to use whatever skill/spell/dohicky i'm wanting to use.
for example, I'm using Feral Swipe (BST aa) on my war, and i just went in and copied another aa based combat ability from the warrior line and changed Gut Punch to Feral Swipe. works well enough for me on RGM. still trying to figure out how to do it in mq2melee for when i'm actively driving but wanna be lazy.