[Tiwaz]

It is possible.

Open \main\pak0.pk3\music\ (I use WinRAR).

You will find .mus files there. You can open them with notepad, to find out what files is being played in which DM map. The files the .mus file is referring to are located in \main\sound\amb_stereo\.

All you have to do is change the filename of your .mp3 you want to be played as background music into the filename the .mus file is referring to and copy it to the \main\sound\amb_stereo folder. I suggest you backup the original files.

You can also try to edit the .mus files, but don't save them in the original pak0.pk3 file.
Extract the files you want to edit, including the folder structure (in this case, extract all mohdmx.mus files and put them in a 'music' folder; WinRAR does this for you by default).
Edit the .mus files, save them, and compress the 'music' folder to blablabla.zip.
Rename the .zip to user_custom_music.pk3 (can be anything, but make sure the filename starts with a letter after 'p'; .pk3's are loaded in alphabetical order) and put them in your \main.
If you did something wrong, edit the .mus again, or remove it from your \main. Good luck.