Sounds like you’d need the curves. I’ll see if I can throw something together. Pretty sure though that the way the spline is copy-pasted inside the blueprint (which is just a list of points) is different from the landscape splines, so don’t hold your breath.
have you considered using splinemesh, export, epoly, quadriangulate, select one of length edges, loop, turn to spline? sure it’s complicated but that is at least some way