Intressthing question, i was trying that myself too, but didn't manage.
So i hope someone knows the answer
Starting a Custom Game


No.
What you can do is to make a series of moves to get your pieces into that position. It might be appropriate to make this a Takeback game (which will be unrated) - as it will be easier to adjust the starting position.
There are over 3,000 starting positions that can be selected - but it will be difficult to establish whether your desired start point is one of these.
(See also:
http://www.chess.com/forum/view/suggestions/custom-starting-positions2
http://www.chess.com/forum/view/general/does-anybody-think-this-would-be-fun
http://www.chess.com/forum/view/general/new-games-with-predetermined-set-up
)
I have seen that I can start a game from a wide variety of opening positions. But what I want to do is create a game from a CUSTOM position.
I have a current game that is at an interesting position. I want to play out the game with a couple variations. I thought I could copy the FEP and create a new board from that FEP and play out the alternative.
Is this possible? I don't care if it's rated or not.