I suggest the new option should be named reuse=<comma separated list of files, wildcards enabled>. exampel  reuse=40000001.img, 40005*.img. If this option is used, then mkgmap should not overwrite the listed folders in the <name>gmap. Naturally mkgmap should write a new tdb, preview.img, mdr and idx. In most cases the reuse folders contains Contourlines or other static content.

Thomas