[Koha] Adding new MARC tag Thesaurus Structure

Dear all, Imagine I want catalog books and save, for each, it's Theme and a Group (these have a restricted set of values we want to be inserted). .I thought about creating two categories in Thesaurus Structure <http://librarian.testes.udp.eurotux.com/cgi-bin/koha/admin/authtypes.pl> named Theme and Group and add for them their autorised values. But, how do I make these new fields visible when adding a MARC and where, in the DB will these values be really stored? Do I have to make links DB-MARC , create new MARC tag... help needed!! Thank you