You need two files:
[NAME].OVR[NAME].RMS[NAME] is a valid DOS file name (minus extension). OVR is the adventure overview (formatted like DUNGEON.OVR).
E.g. If you copy DUNGEON.OVR and DUNGEON.RMS then name it something like MINE.OVR and MINE.RMS, then you can enter MINE on the adventure screen and play basically a DUNGEON copy.
Save games will check for XCH, where X is the first letter of [NAME]. So DUNGEON uses DCH, MINE would use MCH, etc.