Isn't it already present against the Document Type? Check table tfgld458 if it has the relation. If not, you might try adding it through GTM and then use it in your mapping scheme. You can wait for someone who knows a better way to do it. We created a completely new IDT in our case and everything was done through GTM as standard session's do not allow to edit anything.
Unless you try to do something beyond what you have already mastered, you will never grow!