Inserting into HL7 from within BPL

I'm looking at an issue where I need to grab an ID number from an external SQL table when it doesn't exist in a HL7 message, and then add it to the HL7 message.

I have built the logic within a BPL which checks for the lack of ID, and then pulls the ID from the SQL table (and storing its value using the context functionality within the BPL) however I am at a loss as to how I then take the ID and add it to the HL7 message before passing it to a router.

I'm not aware of how I would call the context value from within a DTL (or if that is even the correct way of proceeding) so I'm hoping someone here can point me the right direction.