"It's beyond my understanding" also sounds great :-)

The best way to convey that something is not understandable is to use the words 'incomprehensible', 'unintelligible' and 'unfathomable'. Of course there are other synonyms that can be used depending on the context.

It's not understandable. It's beyond comprehension.

No, you can’t. To be precise, at least, not in formal documents. Even though the meaning is pretty clear, it’s better to substitute it with such words as incomprehensible, unintelligible, unclear etc.