You could try a conversion program iTune Associator 1.6 (freeware) 40k download
available from........

http://homepage.mac.com/josiahpugh/itunesassociator/

or from versiontracker.com

I've used it to convert QuickTime and other mp3 audio files to iTunes, works fine for me;
can convert individual files or all of the (non-iTune) items in a folder...just dump them onto the
Associator icon and they are now iTune items.
Does not solve the "automatic" use of iTunes as a default player for new downloads
but, makes the conversion easier(?).

OSX is not supported by iTune Associator 1.6