You can go back to a previous save file, if you haven't done too much recently. Assuming you are NOT using a Mac, go to C:/Users/[Your Username]/Saved Games/Pokemon Reborn. Delete your current save file, labeled "Game" or "Game.rxdata", and rename the save file that is before you traded the Exp. Share away to "Game" or "Game.rxdata". I'm not too sure on Mac save files, though.
If you did a lot after trading it away, I guess you can post your save file on the forums and then ask for somebody to re-add a Exp. Share into your save file.