Sorry, you can't add a new unit entry, you can only replace a old
here's how:
• find a unused unit or a hero you want to replace
• replace the unit's graphics and iscripts, make sure to use unused flingy.dat/sprites.dat/images.dat entries
• give the unit new commands and make them build-able in memgraft/stargraft/firegraft (firegraft isn't out yet)