No, not asking to bring them to European tree - I understand you had it detailed when selling WoWP to China, some "...and Chinese planes stay where they are..." lines in the contract.
But you can make them accept other nations' pilots, the transfer costs 100k, the usual. The pilots would stay in Chinese planes as long as you need to train them, then transfer back to original nation also for the fee, and we would have the reason to purchase Chinese planes.