If you have AAC format songs that are unprotected you can select them all in iTunes and then choose Advanced -> Convert Selection to MP3.
a) To find your AAC files, you can create a Smart Playlist: Kind contains AAC.
b) Then change the iTunes Import option to 'MP3 Encoder' - it's in iTunes Preferences, Advanced, Importing.
c) Then select the Smart Playlist you created in a), and do the conversion.
iTunes will duplicate all the AAC songs with MP3 equivalents. These you can then move to your music server.