It is possible. You can do it by following way:
Download Chris' OFP Script Editor:
http://www.ofpec.com/ed_depot/index.php?action=details&id=151&game=OFP and install it.
Then run it and go to File - Wizard Created Files and select description wizard.
Then check music and select your music (make sure it is .ogg format), then choose create to disk and go to folder where your mission is (aka C:\Program Files\Codemasters\Operation Flashpoint\Users\Your ingame name\missions\Your Mission\) and click save.
Then again go to your mission folder and create folder named "music" and paste your song there.
Then go to mission editor and your song should be there.