Hi Antje

Just to clarify are you meaning the custom fields were created for the course rather than activities. Course activities are not included in report builder sources and activity custom fields aren't able to be created (grades can be created for activities and total course grade can be included in a report builder report) 

It is possible to aggregate/group fields with the report builder. To enable summing of values the custom field needs to be a numeric field. The fields that are available for course custom fields are text or checkbox so summing isn't possible.

One way around this would be to create a custom field with the menu of choices values and use the count aggregation in the course completion report.

This would count the number of users who fit into each category  for the course and the count multiplied by the value.

Produced by count aggregation on Usersfullname (assuming same userfullname doesn't do the same course)

Hope this helps