Well, first of all I'd better reassure our friend Zongxian that Keymon does
not technically own part of Dankuk!
It is not really feasible to transfer provinces between nations, because the game engine cannot accomplish that. Election results, for example, will always be for the traditional nations, consisting of their traditional provinces. Similarly, nation pages cannot be amended to add more provinces and alter maps.
I would see no objection to a short-term role-play in which provinces are exchanged between nations (eg. as a result of conquest in war). It would be difficult to see that as being a feasible arrangement in the long-term, though.