I would say yes for the following reasons:
1: It reduces the amount of military spending the US has to do to South Korea because North Korea and China would be too scared to fight against a US territory. The US already spends a lot on South Korea. If South Korea were made part of the US, it would scare North Korea to not attack South Korea.
2: It increases the US GDP, which would be helpful in staying ahead of China.
3: South Korea would probably vote GOP due to their bad experiences with communism. To make it fair to the democrats, every US territory would become a state along with part of South Korea.
4: It doesn't have to be militarily obtained. The US can simply tell South Korea to join the US or the US would cease all military funding to the place. South Korea then would join the US in order to keep their military funding. The US should stop giving foreign aid to places where we get nothing in return.