This technically might be okay, although grammatically uncoventional, and a awkward-sounding. The word "much" can be an adj., adv., or noun. If you consider it an adv. in this sentence, then it might be acceptable: "much" is then an adverb modifying the verb "learnt," and "extremely" is an adv. modifying "much." So the sentence would then mean: I very greatly have learned.