Sure, you can fabricate a rack over the AC unit to hold a module. It can be built of off-the-shelf structural aluminum channels. The rack would be attached through the roof decking into a frame piece.
Now, we could discuss the appropriateness of such hardware--In the most typical instance, one module would be mounted like this. Can you suffer the loss of one module? Can a module be mounted either on the side, ala awning type, or could it be portable? I would suggest to think of alternate methods before building a rack over the AC. There are complexity and use considerations that need to be overcome.