hm, i'm not sure if there is an otion to change the starting units, so you will need a new trigger for this, could you describe a bit more precisely which units and how much units you want, and are you just going to change the units for a melee game?
Edit: overload119's trigger would be an option, but as both of us said, describe a bit more pls
Melee maps are kind of touchy. There are very few things you can do without un-meleeing the map. The only way to edit it would be to make an un-melee map with them edited and just put your own AI in (If you want computer players to be allowed).