Go into the group editor and select the piece you want to make a prop and it should turn red.  There is a button to make a new prop and you will be asked to name it.  Do this for each piece you want to make into a prop.  You may want to conform the skeleton to the V3 and make sure everything lines up properly before making the props, but it shouldn't be a problem if you don't.  Also, you can export parts of an object and reimport them.