Hi JaapJan,
I think what you need to do is use QuizPort. This allows you to group Hot Potatoes exercises into a Unit and only have one link on the Moodle course page for as many exercises as you wish. Each QuizPort has just one grade in the grade book.
Here's an example: English for Beginners. Behind each green "tick" are 10 to 20 exercises.
Cheers,
Glenys