What do you mean, "the same way they retook Hong Kong?" The United Kingdom had a contract with China regarding Hong Kong, and the City reverted to China in 1997 as part of that contract. For better or worse, the U.K. voluntarily ceded Hong Kong back to mainland China as part of an agreement struck a century before. Is there such an agreement between China and Russia? Hong Kong, including Kowloon south of Boundary Street, was ceded to the Crown in perpetuity. Kowloon north of Boundary Street was leased for a century.
Well, then why did the U.K. give it up? It's my understanding that they tried to negotiate continuing to administer all of Hong Kong in the 80s, but China balked. I guess the U.K. felt splitting Hong Kong was not feasible, and decided to transfer everything.
At any rate, China did not take it by force, and the U.K. did it voluntarily. If they hadn't decided to transfer back, would China have tried to take it back by force. It seemed as thought the majority of the population was not in favor of the return.