Henk,
Since I don't know much about what you are trying to do, I am making some assumptions and some brainstorming. I am assuming that you want a certain amount of credit to add to the tally for each quiz. We used to have an aggregation type which was sum of grades, but I don't see it any more. My suggestion would be to make each quiz worth the number of "credit points" you want it to count for. (If that doesn't give you the points you want for each quiz, you may need to manually override the grades. ) Now go to the grade book settings and create a category in the grade book where you will put these quizzes. Then for the category total, edit settings to be the adding together the grades in that category. Here is information on grade calculations. You will want to go to the course total and hide it so that you don't have problems with students seeing another grade aggregation.
I hope that makes sense.