Over 14,000 Chinese POWs refused repatriation to Mainland China and went to Taiwan instead, according to Jeremy Brown. Over a million Chinese were sent to Resist American and Aid Korea, and a many of them would have served in the Nationalist armies. Peng Dehuai, the Chinese communist commander in Korea had been in the Nationalist armies during the Northern Expedition in the 1920s. Many Nationalist troops defected to the Communists during the Civil War period of 1945-49, and some of them probably ended up in Korea. There were a lot of people who were formally part of both armies at one point or another.

Brown mostly looks at those were were shanghaied (sorry) into the PLA in the Southwest in the early years of the PRC. The takeover in the Southwest was rather slow and chaotic, and there was considerable armed resistance to Communist rule. Brown relates the story of Li Huagao, who apparently had nothing to do with the Civil War before 1949 not surprising since he was from a peasant from Guizhou, which was far from the areas of active fighting. He did join the resistance to new Communist policies after 1950, however, and was captured, put into a labor battalion, and then eventually sent to Korea, all without ever having had much by way of political ideas. Hu Zhengming, on the other had, had two years of education in Sichuan before joining a Nationalist youth battalion at 15, and serve nine years in the Nationalist forces. He was in command of a security unit in the civil war, but lied about his rank when captured and claimed to be a common soldier. He was put in a mess unit and sent to Korea, where he promptly deserted. Sometimes whole captured units were sent to Korea.

So, in short, there were plenty of people serving the PLA in Korea who had at one point been in the Nationalists armies. Some of these, like Peng Dehuai, had done so many years before, during one of the United Front phases. Others were drafted into one army and then the other without ever having been much concerned with politics, like Li Huagao, although there were also others who had genuinely changed sides (and might do so again), or were genuinely anti-Communist. Any of these might have ended up in the group that went to Taiwan.

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Source

Jeremy Brown "From Resisting Communists to Resisting America:Civil War and Korean War in Southwest China, 1950–51" in Brown, Jeremy, Paul G. Pickowicz, eds. Dilemmas of Victory: The Early Years of the People’s Republic of China. Cambridge: Harvard University Press, 2010.