I would like to play against the computer from certain positions in my past games, for example I would like to start playing with it being my move 11 from this game
Is that currently possible, and if so how do I do it?
if you use fritz, it allows you to cut&paste a game in - select any move number and continue the game after that. Often i do this inadvertently - I really meant to enter a variation and instead the computer starts playing me.
with chessclub.com's client (available without an account), you can enter in so many moves and then just make the computer think about the next move.(crafty) This would work for your purposes - you'd have to make the move the computer selected manually.