How to seamlessly add extra tiles outside original extents?

I’m using UE4 and World Composition and I’m trying to figure out how to add another set of tiles seamlessly to existing terrain. I’m able to drop in new tiles in UE4 and match them up of the same resolution, but how could I go about extending the extents in WM while maintaining the original tiles positions and adding to the ‘boundary’ without messing anything up?