I've just realized that I don't know how to do this myself, so I'm interested in the answer as well. I know that vehicles can be spawned via the spawnvehicle command, but I'm not sure about bots. The difference is that a vehicle is not controlled by a bot AI by default, but you'd want that with a character. Also worth noting that whenever bots are involved, the level must have been exported with pathfinding pre-generated so that the bots can find their way around. I'm not certain, but I would suspect that this is the case in all SP levels, since fighting the bots is the whole point. But in MP, they will misbehave because they have no idea where to go (I've seen this when spawning them by other means).
Also, Hi there! I see that you are new here. I'm going to move this thread to the appropriate subforum.