It's surely possible, though it involves start pos editing I think, so it's probably not that easy...
One thing I can tell you, though, garrisons are almost worthless (imo and in the hand of the AI). They don't have the combat power of regular units. Don't be feinted by their numbers, you can crush them easily even when outnumbered.
Here is a screen to support my statement, numbers are almost equal and my army even consists of rather crappy AOR units mostly
Sorry that I can't be of further help, creating such a mod would excel my capabilities.