Not sure what you want to do and what you mean by "costum crest".
Concerning crests, it's possible and it's not... It's complicated... You can give any team any crest you want through the Kitremapping table (give a volta crest to Liverpool for example). However, it will only affect crests on the jersey and on the team select panel.
There are so many instances where the original crest will appear instead of the one you want to use, especially during a match if you use "pause".