Hi Uli,
The Moodle Events API documentation should help here. You can pass your own SQL query with get_records_sql
$DB->get_records_sql($sql, array $params=null, $limitfrom=0, $limitnum=0)
You'd be looking at the mdl_user, mdl_user_enolments, mdl_enrol and mdl_course tables I would have thought 
Hope this helps