This simplifies an extraordinarily complex question in Chinese history.
At times, of course, Chinese citizenship has been roughly equivalent with (Han) Chinese ethnicity. This was, of course, a considerable factor in the revolution that overthrew the Qing Dynasty, as the Manchurian Qing became viewed as foreigners oppressing increasingly nationalistic Han Chinese. But, of course, for the first couple of centuries of Qing rule, their non-Han ethnicity was not problematic whatsoever.
More currently, consider that contemporary China is home to fifty-five official ethic minorities, aside from the Han, comprising approximately ten percent of China’s population. While the disenchanted Uighurs of Xinjiang often receive the most attention — as they allow journalists to combine the “rise of China” angle with a “global Islam” angle — Chinese ethnic minorities enjoy, for the most part, a comfortable existence within China.
So to return to Zen Pundit’s original point, China may have no problem whatsoever accommodating new ethnic minorities, seeing as how it has had little problem accommodating its existing minorities. Look at the growing population of (South) Koreans in China, especially in the northeast, with a population reportedly approaching one million.