There is Tibet, which had declared independence in 1913 and then decisively split itself from China by expelling all Chinese in 1945. The PRC conquered them because they were part of the Chinese "motherland". You can argue that the communist were the lesser of two evils (I would agree), but you can't argue this wasn't an imperial conquest.
The situation in the rest of China is a lot more complex. Most of warlords joined the PRC (sometimes through negotiation, mostly through surrender) when it was clear that the ROC - the competing but less centralized imperial power - had lost. The program of Han settlement, Sinicization and ethnic repression that occured in multiple waves (most acute in Xinjiang, Tibet, Inner Mongolia and some areas of SW China) was an imperial project.
Externally, there is the Sino-Vietnamese war of 1979. While the reasons for the conflict were multi-faceted, one of them was that Vietnam had broken Cambodia out of the Chinese sphere by deposing the Khmer Rouge.
Recently China has been building bases and shaping countries' economies and political systems around the world. Arguably they have already made the Solomon Islands a protectorate and a few African countries are also moving in that direction.