I'm looking for a Linux application which can add a new tag to a mp3, like foobar2000 does under Win.
Command line apps like id3v2 can only manage already present tags.
Any clue?
Many thanks

Try eyeD3 or python module mutagen.

If you like foobar's masstagging, you'll probably like Ex Falso (which is the standalone tagger that comes with the music player Quod Libet).

http://www.sacredchao.net/quodlibet