Now, add bots to the team you want to. You are right, some of them end up in spectator mode, but do not mention that yet. Just keep adding the bots.
Now, when all is set up, you see probably 2 or 3 in the opposite team and the others are spectators, type this in console:
sv_restartround 1
once the round restarted all other bots are joined to the other team. Atleast thats how it works for me and my bot, i did not try this 'trick' on other bots.