Yes. Try /addbot in the console.

For example: /addbot Ranger 5; loads a Ranger bot on the Nightmare skill level.

If you want random bots to spawn, specify the skill level on the server setup menu and set minimum players (or /bot_minplayers in the console during a game) higher than 1. Like if you want 5 random bots, use /bot_minplayers 6. You add one to the number of bots you want, because that counts yourself as a player and the command will spawn bots until that number of players is reached. If another player joins, one bot will be kicked.