Adding a jib would definitely upset the helm balance, plus the whole point of the cat ketch rig is not to have to worry about jib sheets and bringing the jib across, but just putting the helm-a-lee for a leisurely tack.
If you like the spritsail's low center of effort and gravity and short spars, but perhaps want to avoid some of its disadvantages, I would suggest you go the way I did with my self-designed cat ketch: use two standing lugs with sprit booms. AFAIK it may be the only one of its type, but it's stable, low center of gravity and effort, traditional, nicely short wooden spars, easy tacking, not too difficult to reef. Initial rigging up is a little tricky, though. More details on http://176inches.blogspot.com.
But I would not mess with the sail plan of a professionally-designed boat without very careful analysis of Center of Effort and Center of Lateral Resistance.