Sure. Just don't send the score until you have an average: keep all attempts in Storyline variables, then calculate the average, and send that number to the leaderboard.

Thanks a lot.
The next problem is, that next time (after leaving the quiz) the same user try the quiz, a new row will be added instead of updating the existing row.
Is there a solution to that?

This is how the widget works. Every time you call the code to add new data it will add a new row. The solution would be to not call the code until you are ready to save.