in the force (wherever it is - probably gst) you just need to make another force as a child to it and link all the same categories. after that you just need to make sure that the parent force's categories constraints are checking child forces
hmmm, have to test that and see if the child force can modify the category requirements of the parent if it's setup like that. I'm going to guess that 40k would not, but does 40k have an army with anything that would modify the Batttalion (using that as an example) requirements?
@WindstormSCR Thanks. I was thinking rites and legion selection would have to be in the parent and just have the legion specific units in the child. It should work out but we'll have to give it a go and see.