It is possible to take the exams even if you’re not learning with the Cymraeg i Oedolion system. I took the Uwch one a couple of years ago. I did go to a revision day to help me with exam preparation (but found it quite dispiriting as the tutor didn’t quite have the same approach to mistake-making as SSiW and spent the whole time interrupting me to correct my treigladdau).
If you do decide to go for it, there should be past papers (or sample papers) on line for you to practise on.