Changing the militias is a bit more complex, but changing the Onderon soldiers/officers isn't so much.
It's mostly copy pasting values in appearance.2da (which you can edit with KotOR Tool).
For example:
row 355, label Republic_Soldier_Mal_w_Helmet, normalhead: 58, modela: N_RepSold, texa: N_RepSold
row 607, label Onderon_Soldier_Mal_Masked, normalhead: 146, modela: N_OndSoldM1, texa: N_OndSoldM1
Leave the row and label intact, copy the rest of the values above from the Republic soldier to the Onderon Soldier.
There's only one Republic Soldier with a helmet (there's the female one but there's no female Onderon soldiers), so if you replace the few variations of Onderon soldiers + officers with helmet guy it will be Iziz: Attack of the Clones 🙃
Edit: I'm adding a text export that could help anyone who is up for doing this switcheroo. I'm not myself, sorry.
appearance.2da.txt