Well, you could manually make it (look up fen on wiki it's pretty simple)
But much easier is, (you mentioned you had winboard) in winboard go to
Mode -> edit position
Then rightclick on a square opens a menu of all the pieces. When you're done
File -> Copy Position to Clipboard
Saves the FEN to your clipboard. Then you can paste it when setting it up in chess.com.
Invalid FEN


Thanks! Turns out you can do it on chess.com to at http://www.chess.com/analysis-board-editor
Is there a system I can use to create invalid FENs, specifically one in which there are more than 8 pawns of 1 color?