I don't think that you can get the harpoon on other levels, it might be scripted. You could try the give weapon command code and experiment with what you call the weapons. But it didn't show up when I did the impulse 101 cheat so it might be scripted...
I can't figure out how to give the spawned units weapons... D: So when I make them so its combine vs antlions vs zombies, the combine just kinda stand there... :dozey: