If there is a college class named "multibody dynamics" or similar, then it will be the bare minimum that you need to become expert. By itself the class can get you to the "functional" level from which you just need experience to get "competent".
I understand you cannot actually attend classes :) but that does not mean you cannot do the course.
You want to be an "expert" and that means being prepared to go the extra mile. If you just want to know about it as a hobby, that is a different proposition.
Find out what the textbook is and see if you can get hold of the regular assignments... use your initiative.
There will also be other courses that go with that one - find out what they are: the college prospectus should help - they are online these days. You could also contact students, find out who takes good notes and ask them to get you a copy.
A lot of colleges put their lectures online - have you looked?
If you are not prepared to put in this sort of work, then you probably won't become "expert".
But you could become "knowledgeable".