Same way as West Germany welcomed East Germany to the free world and the rest is history and we ate hear to talk about it.
West Germany welcomes a 1950s-1960s society - East Germany into a 1990s world. That part of Germany is still a drag on the German economy plus it brought in Merkel. I am not saying it was bad and they shouldn’t have done it. Just pointing it was not easy and it wasn’t cost free.
South Korea will be integrating into its society a functionally a medieval level society, where the average North Korean of military age has the physical (and probably mental!) development of a South Korean 12 year old. This will be a huge huge challenge! That’s why all the SK’ers I talk to aren’t that enthusiastic. The ones that are living here in the US tend to be the more entusiastic, they won’t have to live with & pay for the process. Many SK’ers in South Korea actually prefer the status quo as sad to contemplate as that is!