What you're asking is like "can i extract the source code from a compiled program?"
Well, of course not, you can't do that.
If you're looking for the .qst file to open with queditor, then you can't get it back.
Anyway, there's still something you can do even with a compiled quest: you need Cv5, a program that lets you "see" inside the .mtf files (it was originally made to extract .tga images) -- see the link on my page (mh, i just noticed that i have a double link to that page... well, i'll fix it later
)
Now,
modifying an existing quest could be very dangerous if you're not familliar with queditor and how DS runs custom-made quests.
Basically, a .mtf file is just an archive: opening it with Cv5 lets you see the inner directory structure, and what files are inside it. This way you can recreate the same directoy structure within the DS folder, in order to have it override the content of the .mtf file.
Malleus