As far as I've seen, this game isn't supported anymore. And as far as I know, Ubisoft hasn't released an SDK for this great game yet. Now, I was wondering if there was a way to extract the files and edit them?

It depends on the meaning you give to the word "edit".

Actually you can:
edit textures,
edit monsters/player's RPG values such health, strenght and so on

partially substitute monsters with other monsters with the same animations
partially mod some map entities value

You cannot edit maps nor create any new either

To extract files search Nem's GCFScape,
VTFEdit for textures, ConText or notepad for the QCT files, a good Hex editor for BSP files

Well, I meant like modding and stuff. And thanks for the info, I'll check it out.

So, we can't add new monsters and spells and stuff like that?

If you decompile DM mosters in order to know what their animations are named you can recompile monsters from Source games almost as well then, after renaming them, you could spawn them in DM.

Though they will simply act like DM monsters, 'cause if you want them acting different, you must deal with code, thus with the damn-never-released SDK...

The same with spells.
Though you can edit some spell values editing the spell.txt file contained within the mm_engine_pub.gcf file

PS
You do not need to edit the .gcf files.
Simply recreate the folder tree under the "dark messiah might and magic single player" folder and put your edited files in the proper folders

Ah, I see.
So, if I wanted to work with the code, what would you suggest I do?

Thanks for all the info.
You're alot of help Fender

Well if you're really smart dealing with code, I'd suggest to deal with HL code.
It is available, and you'll find lot of literature about it (the mod forum here has threads about tech support mainly ); plus you could borrow monsters and map models from DM and set them in HL maps.
In addition you could find other people interested with it, and push an HL mod similar to DM (this would be not so different from what Arkane made indeed, but please, do it for free this time )

Haha, I guess you're right.
And I was thinking of probably doing that...but wouldn't Ubisoft get all ****y about the whole ToS and EULA? (Even though it IS for free )

...oh the EULA...
I guess you've been lured with the word "Source" within the game, I have been too.
Right, to hell with UBI, DM and SDK...

We just have to find some other guys happy to modify Alix into a girl NOT named Leanna and HL zombies into ghouls and undead and we're half way done...

Haha, don't be too hasty to discard the hope of modding there my friend.
Who knows, maybe one of us'll find a way?
As long as there's people, like us, willing to make this game better than it is, then we'll sure as hell do it.
Not to mention that this was a really poor decision on Ubisoft's end.
They basically destroyed one of their only innovative and captivating games, by ending support and not giving modders the ability to fix what they left (and enhance what Ubisoft has made )
Or maybe we should just wait until it becomes freeware...