Sure they can be done. Examine the mission lua, the number and type of player characters are set there.
The ReadDataFile sets up which meshes the level will need:
so if you want to add something new, make sure it exists in the game's side lvl file, and add it in to the appropriate ReadDataFile.
Next look at Alliance / Imperial Stats, this defines how many of each type of unit will spawn:
-- Imperial Stats
Then, check the Attacker / Defender Stats has the same total number of units as called in the above section:
-- Defender Stats
If you want to change their weapons, you'll either have to hex edit the game lvl files or do this