I don't understand your problem. Take a free audio converter, convert your mp3s to oggs, and that's it. I don't think that the quality loss is noticeable for normal people. What does "apparently it's not optimal" mean? Do you have a real problem?
And mp3 is lossy, ogg is lossy, so yes the result is lossy. If you want a different result then use a different format. In any case, I don't think that the conversion process has anything to do with the result (as long as you don't use a crappy software), only the chosen format is relevant.