Which FREE software can I use to play with computer from a certain position on?

So... There are lots of softwares which either let you make human x human games or human x computer but they all need the game to be played since the beginning with that configuration, but Ido not want that...
I want to set it human x human then play the way I want until I find a certain position and only then set to human x computer and let the computer play from that position on...
Which are some FREE softwares can I use that allow that?

You can use en.lichess.org/analysis. Play your moves, then enable practice to continue against the computer. It's the target button on bottom right side.

Well you can play as you want for both sides in analysis board then you can switch whenever you need with "continue from here > play with the machine" option in menu. If you want software anyway SCID, ChessX and many of free option are avaiable to do that.

Yes, in Scid it is 'Play' -> 'Serious game'. It has an option 'Start from current position'

I use the demo version of Rybka to analyse some games, but mostly I use the great Lichess.

Thanks @thibault and all of the developers here for making this site so awesome!

You can also set up a position on en.lichess.org/editor and then press "continue from here" ---> "play with the machine."