The way to do that it to achieve independence for South Korea. The North is clearly already independent. So the US forces must de removed from South Korea and and having them install the THAAD system is a step in the wrong direction. South Korea might declare itself independent and ask neighbouring countries for guarantees but how would US react to that?
After South Korean independence the two parts can start a reunification process that might take years but that would include early on the building of rail, road, pipeline and glass fibre connections between the South and the North, China and Russia.
Yes, for South Korea to become independent, they would need to terminate the SK-US military alliance. But that carries all sorts of risks as well. The only way for them to mitigate this would be
a) a security guarantee from China
and/or
b) a concerted increase in military spending, so that South Korea can stand on its own without outside assistance.
Course A is difficult given the existing China-NK relationship, but not impossible given the contempt that China holds for NK.
Course B would face a lot of resistance, as there are many interests in SK who are happy to underspend on the military. SK really should be spending around 5.8% of GDP on the military like Israel or UAE, which would be twice what they spend today.
---
And if SK asked China for a security guarantee, the US would not react well at all.