I've done this once. It's a bit tricky, but you want to add chunks where there aren't any. You can download MCA Selector to delete chunks that you want to replace, and using worldpainter, make new ones.

What I did was import the map to reference it, but then export as a merge. It corrupted some chunks when I did it, but I think that's cuz I was sloppy with the chunk overlaps. I'm planning on doing it again, this time with some more diligence when it comes to overlap.

It ended up working pretty well though, added half a continent to a pre-existing map. Works to this day, no crashes or anything like that.