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"

By analogy to "crashing a party," here "crashing a course" means attending without being registered. The Oxford Learner's Dictionaries are an excellent resource for English learners. https://www.oxfordlearnersdictionaries.com crash - verb https://www.oxfordlearnersdictionaries.com/definition/english/crash_2 definition 6 party ​[transitive] crash something (informal) = gatecrash gatecrash - verb https://www.oxfordlearnersdictionaries.com/definition/english/gatecrash ​[transitive] crash something (inBrE /ˈɡeɪtkræʃ/ ; NAmE /ˈɡeɪtkræʃ/ (also informal crash) Verb Forms ​[transitive, intransitive] gatecrash (something) to go to a party or social event without being invited