[@Duthguy]  I would say no.  A race without a natural transformation cannot just make one up; however, there are plenty of ways to essentially reach a level of transformation and even change appearance.  A player could learn specific techniques like the Kaio-ken to boost themselves.  Certain types of training can lead to semi-transformations, such as learning how to create God-Ki which could change your aura, reaching a calm state of mind similar to ultra-instinct, learning a technique to gather a ton of energy at one time to power up like absorbing a spirit bomb.  One could also go a cybernetic or genetic manipulation route if they so wish, the possibilities to attain a 'power-up' are quite numerous so be creative with it.

Now if one makes a magic based character, unnatural transformations could be more of a thing since magic is completely different from Ki.  Magic is its own beast so pretty much anything can happen, within reason of course and any transformation will have to go by me first.  I feel there are plenty of routes you can take for 'transformation' purposes.  While I don't want to limit you too much, I don't think it is a good idea to just let any and all races suddenly have transformations, needs to be a racial trait or some kind of system based on the DBZ universe, be it games, manga, or anime.