Can you do this in a thconfig:
source "cave.th" source "extend1.th"
where cave.th contains the cave survey data, and extend1.th contains the "extend" commands? I would imagine that to do it, you would need to re-enter the original survey, which is not permitted.
Multiple source files are fine. I have a project (more than one?) where I do this routinely. It is a nice way to toggle the scope of a compile up or down without having to dig into maps. I have not used it specifically for extended elevations, but there is no reason I can think of that would preclude this use. It might even be a great way of doing it. There is no reason you would need to re-enter survey data for this to work, and as you imply it would be extremely bad form to do so. Bruce