iTunes/Mp3-Player and actual multi-tagging of MP3s I search for a software to allow multi-tagging.
Meaning, i want to add more than one genre to a song/album to be able to easily access specific songs i.e. ("female singer, jazz" or "duet, jazz") which i can not realy do by choosing just one genre.

Is there an option to do this with iTunes?
If not, is there a player able to do this?
Maybe even with pandora/last.fm/musicbrains interface so i don't even have to tag all the songs myself?
For a thorough analyse of the topic, read Ways to store multiple genres in your digital music, where Dan says:


  MP3. MP3s (or rather, the ID3 tagging format used by MP3s) don't support multiple tags for genre. Instead you must useone existing tag and use a delimiter character to separate the genres within that tag. 


If you use iTunes player, I'd recommend to use the Grouping tag for that purpose.  

Now, can you do that automatically ?
As you pointed out, Last.fm offers an interface to retrieve tags for songs.
And bdefore used that API to develop an iTunes Multi-Genre Tagger (description blog post here, last version of the script here). I think that's the tool you're after.