I am not clear what, "I'm crashing this course now" means. Maybe a bit of context would make it clearer.
There is an expression - to take a crash course meaning to take an accelerated course. I suppose it might mean that they are speeding through the class.
It could also mean that they are failing as in "crashing out".
You can sit in on a class if you are not registered, but you find the subject interesting. An observer might sit in on a class to asses the quality of the teaching.
So the expression is "sit in on"