While I am aware you can import a PB20 League, I want to know if it was possible to transform previous saves into custom roster files. In this case, I want to be able to have the same historical data as before, but start with a new team. Any suggestions are welcomed.

Because of changes that went into the game its really not an easy process to take any kind of previous save and turn it into a custom roster file. It can be done and I can give you instructions but it is going to take some significant work on your part in creating files and testing them.

That's totally fine, thanks for the heads up. I'd still love to receive instructions and try my hand at testing them out.