Okay, I solved it myself:
1) Add in src/thread_view_common.inc.t in the database call funktion "make_perms_query" (Line 42 in FUDFORUM 3.0.6)
", f.descr"
so roughly:
... SELECT f.id, f.name, f.descr, f.thread_count ...
2) Add a tag in the Thread-Page (I added in header.tmpl for all pages, so I had to add a check to just show it when there is a thread description:
{IF: !empty($frm->descr)}<div class="headtitle"><h1>{VAR: frm->name}</h1></div><div class="headdescr">{VAR: frm->descr}</div>{ENDIF}
3) Add a tag in the TITLE section, my title is now
<title>{GVAR: FORUM_TITLE} {GVAR: TITLE_EXTRA} : {VAR: frm->descr}</title>