1.First of all you need an editor.
I prefer "St0rmcat's MM7 View" which can be found at http://www.angelfire.com/id/mmvii/files.html

2.Start mm7view then -> file-> datafile -> your might and magic 7 data folder -> open "events.lod".

3.Open Mapstats.txt and replace the monsters you like.

"example" goblin for firefly

Look at the emerald Island line -> change Firefly to Goblin
and save. then import the mapstats.txt.

4.You could edit the monsters stats in monsters.txt
but you dont have to ofcourse.

5. Start a new game and go to the "firefly place"
now you see goblins!