Sure, you can do basic extension like you can with Unity if you have reasonable understanding. But no one man bands or even teams will ever understand everything about how the core engine works in either UE or Unity, because you’re essentially trying to reverse engineer 100’s of engineers work over several years which is impossible and impractical…
So if your plan is to make a game, I wouldn’t even try unless there’s a problem that needs to be solved. Games are hard enough, never mind anything else…