Can we copy documents after objects being created?

Summary:

We have a case wherein we are copying specific documents to invoice and specific documents to bill from buy shipment, right now we are copying documents considering invoice and bill already exists against buy shipment but now we have a case customer said we will upload documents on shipment regardless invoice or bill exists or not so we want to copy specific documents after object creation

For example - we want to upload POD to shipment and that POD should be copied to invoice but user is uploading POD before invoice creation and wants that POd should be copied after invoice is created against that buy shipment