Getting the base64 for a given document

Is there a way to get base64 for a given document?

I have a Expression rule where one of the rule inputs is a document. I'm using this Expression rule to prepare the body to be sent in one Integration object (we need to execute some logic, and that's the reason why we are doing it in one ER). How can I get the base64?

We know that we can manage it by adding a rule input in that integration object, and passing there the document which will be automatically converted to base64, but the approach mentioned above is possible?

Thanks in advance