I think it could be done, but nothing would be done automatically. You'd have to manually build a new fig file. Use the m-file to give the controls you add the same tag name as is used in their create/callback functions.
You'd have no way of knowing of any initial values that were present in the original fig file. nor what strings or values were prepulated in popup menus, listboxes, etc. Basically anything that was set in the fig file that does not appear in the .m file.
If you are familiar enough with what the gui did, you could probably recreate it.
