Well, just to be a pedantic sod, but you can indeed have a ski jump and a catapult on the same carrier, just not both at the same position.
It would certainly be possible, even desireable for the PLAN to have waist catapults and a bow ski jump on the same carrier.
China has already invested in 20+ J15As, and will probably get many more to form the Liaoning's air wing. That is no small investment.
They are also working on a catapult capable J15B.
A waist cat mod could very easily give the Liaoning the full benefit of cat assisted launches for strike loads and especially fixed wing carrier AWACS, while retaining the ski jump allows them to continue to use the non-cat capable J15As.
Once all the J15As are retired, they could do a major mid life upgrade to remove the ski jump and put in bow cats, but that's at least 15-20 years down the line.