It should be relatively easy to add in... there's a question from a few weeks ago about having separate sprites for the front and back that goes more in-depth, but here's the cliff notes version:
- monster_get_battlesprite is used to get the monster sprites, this uses the AMP ID of the monster to read sprites from so you could read its gender and then return different results if the species has separate male and female sprites.
- These sprites would need to be added in init_monster (instead of just a map and battle sprite, you could have map + battle male + battle female, or even separate male and female map sprites), and of course this also means you need to modify init_monsters so that it gives the correct number of sprite arguments in each init_monster call.
To avoid having separate handling for monsters with and without gender differences (which makes things complicated...), I'd just have them always have separate male and female sprites in the code, but add the same sprite twice if there's no gender differences.