There is a special category for partners of citizens who have lived outside New Zealand for 5 years or more. If your children have been overseas for a long time then there is a chance their partners can apply directly for permanent residence (영구영주권). If they fall into this category then they can save a lot of time and cost. You should contact a professional adviser or lawyer to discuss.
Anyway, whether residence (영주권), or permanent residence (영구영주권), they can apply while they are in Korea. Again, I suggest contacting a professional adviser or Lawyer to discuss the details, because it will depend on their personal circumstances.