Er... FS is hardly the easiest game to mod, but it's as good as things come in other respects- most moddable games (a disturbing minority in the gaming world) have one of two basic approaches to modding:
1. You can make some things (usu. player units) look different, and occasionally change the damage capabilities of units, all using methods a 6-year-old could sort out, but cannot add or change the properties of existing units.
2. You can do whatever you damn well like, including modifying AI, completely changing units, writing new missions with complex scripts, but you have to be a programming god, and probably godly in several other fields.
FreeSpace is a nice medium, which is rare. Be glad it's not like most.