Wouter,
It should be possible to add the column you want with a bit of coding - the best place to put it would be the add_course_tables_to_joinlist() and add_course_fields_to_columns() functions in local/reportbuilder/classes/rb_base_source.php.
That would make the new column available to any report source that includes the other standard course options.
There is some documentation on building report builder sources that might be useful here:
http://docs.moodle.org/dev/reportbuilder
The only issue I see is that you may end up with multiple records if a user has multiple roles in a course. That could be solved either by creating a join made up of an aggregated subquery, or by using the one of the text-based grouping options like rb_group_comma_list().
I'm happy to advise if you need more help - and if you get it working please share and we'll look at including it in core.
Simon