Exporting my iTunes playlists to MP3 to add to car audio system...

Each time I think I have the problem solved , I don't.

I have been using iTunes for over a decade, so some of my library goes way back. I recently purchased a car that has a hard drive music system but not an iPod interface, so the only way to get music into it is to transfer it via USB stick. The files have to be in MP3. I apparently have not jumped through enough Apple hoops to convert the songs I purchased into MP3 -- I get a message that tells me the songs must be in iTunes Plus. Can I convert my whole library into ITunes Plus? I just want to use the music I purchased on a system that I own. I resent being treated like a potential pirate.

What is the solution, if any? Thank you.