Is there a way to add bots in a multiplayer deathmatch game? I don't see an option to do that in the Multiplayer menu.

Short answer: No

Long answer: That was never added to the game. Adding it to the game would require a lot of work to implement the necessary AI, paths, and path nodes necessary for bots.

Here's a link to all the known mods and tools for Chaser:

http://www.chaser-game.net/?page=downloads

No, it is not possible. Bots can be only in the singleplayer.

Aww, that's too bad! I really liked the engine Chaser used.