MDK 1&2 are now supported on Xfire (although both need to be manually found), but MDK needs a tiny bit of fiddling.
Rename the MDK.exe to MDKD3D.exe and then manually search for the game in Xfire.
The desktop shortcut will need to be changed as well, unless you just launch via Xfire all the time.

There is another way which don't require renaming of files. In Your Xfire directory find "xfire_games.ini" and edit it. Inside You should find (use [Ctrl]+[F]) fragments about both games. Now change the value of this little variable to the name of GOG.com's .exe file:
LauncherExe=MDKD3D.EXE
You can also change this one:
LauncherDirKey=HKEY_LOCAL_MACHINE\SOFTWARE\Metaboli\MDK_Collection\PATH
It shows place in the registry where path to the .exe is stored. Click Start Menu in Windows and use "Run..." option. Write "regedit", push [Enter]. Search for:
HKEY_LOCAL_MACHINE\SOFTWARE\GOG.com\Folder_of_Your_Game\PATH
Now just copy this directory to above variable and Xfire will find the game now during installed games search.
This way You can add any game to Your list that is supported by Xfire but program can't detect it correctly. Reason? Very often Xfire searches only for... Steam version of many old games. Don't You think we should spam them for adding GOG.com's games too? :P
One more thing - make copy of the edited fragment of the file. With every update of the list on Xfire it will be overwritten.