Can a 12x142 rear hub and 15mm front hub be converted to 9mm quick release with an adapter?
YES. 
You will need a adaptors called "end caps" that fit the ends of the axle (front&amp;rear) and make it compatible with the use of QR. Depending on the brand of the hubs, the "end caps" can be found on the market - otherwise will need to make it at a turnery (it's a simple work).
But there are exceptions! Some front hubs of type "Boost" are 15mm but slightly wider (110mm vs. 100mm) than traditional QR and 15mm hubs. If the wheels you buy come with this hub, it will not works even using "end caps". 
Before buying future wheels, make sure there are available in the market "end caps" for them.