There's an even easier way, which doesn't involve editing maps - you just use console commands to change the gametype. First you need to set up an admin password for your machine - to do this open up the console (~ or TAB) and type
preferences. Then click the GameSettings tab and add a password to the bit at the top that says
Admin Password - eg. llama
Now, start any practice game, and before the game starts bring down the console again and type
adminlogin llama (or whatever password you chose). It should say you have become a server administrator.
Then, to play face as a DM map type :
admin switchlevel ctf-face?game=botpack.deathmatchplus
This will change the map to Face, but you will be playing it with DM rules. You can also use any of these game types :
botpack.teamgameplus (Team DeathMatch)
botpack.Assault (Assault - dunno how this would work!)
botpack.Domination
botpack.ctfgame (CTF)
botpack.lastmanstanding
You can mix up any of the maps with any of the gametypes, but some will result in strange games