That too. But that pretty much nullifies the whole idea, doesn't it?
I don't see how practice maps would help you. If you need practice with your aim, go and download OpenArena and play that.
I started out with games like Quake 3 and Unreal Tournament, so all I really needed to learn when coming into Trem was the game mechanics and how to play as aliens. Single player alien practice would never work simply because you need other people shooting at you to really know what to do.
Now, I don't think that practice maps are a bad idea, I just think that single player practice maps are useless.
And anyway, for the ultimate training exercise, you really need to go play a game with others.