Combine exercises from different modules into custom mastery challenge

First of all, I'd like to thank you guys for Khan Academy. It's a wonderful resource and I've used it a lot to teach myself about different maths subjects.

I'd really like the ability of creating a 'custom' mastery challenge whereby the user can select quizzes from different KA topics. I know this feature sort of exists for classes, but that's not practical for those of us who do self study. This feature would be useful if you want to keep practicing material from a variety of subjects that you have already finished.

EDIT: would like to add that in some of my university courses I need to practice materials from different KA units (e.g. calculus + complex numbers).