Don't post songs at all since it would still be file sharing, regardless if you acquired these songs legally or not. Unless of course these songs were not copyrighted to begin with, for whatever reason.
There are hundreds of free MP3 and WAV to OGG converters available on the internet. Its probably quicker to find one than it is to send an mp3 and wait for an OGG to be returned. There are many that will do this directly from CD which means you dont have to convert to MP3 first. Can I remind you that "backups" of orignal CD's are still a grey area! Only your own materal and what you have writen consent for is strictly legal to copy or reproduce.