The page WinsingtonIII linked to pretty much spells it out, just put the command into the genericScript and fire it up. Please note however, that they will be a recruited general, and not a family member. If you want to make Hannibal Barca a family member, the best way to do this that I know of is to spawn him as a rebel general with Carthage as the sub_faction, then bribe him to join your faction, but this requires a bit more work. I can give more detailed instructions if you like, as I have a fair amount of experience fiddling with this sort of thing.