Simple tutoriak using only blueprint that bakes a Spline Mesh to Nanite Mesh, and vice versa, Nanite Mesh back to spline.
Simple steps;
-Select Nanite Mesh, make a temporary copy and turn nanite off.
-Use the temporary mesh as base mesh for the spline mesh.
-Bake the Spline Mesh into a single Mesh using blueprint and Turn Nanite On.
-Delete the temprorary Mesh.
https://dev.epicgames.com/community/learning/tutorials/9GnP/unreal-engine-convert-spline-mesh-to-nanite-or-nanite-to-spline-mesh