[Koha] Adding new MARC tag Thesaurus Structure

[Pedro Faria]

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