No, not using the rubber stamp, just dragging the PDFs in.
Let me rephrase:
How do I create an ADA compliant PDF (machine readable text, alt-text for images) in OG, considering also that some of the images are PDFs that I’ve dragged from an external program onto the canvas?
This might be one to directly ask Omni Support staff since it deals with how information is exported and placed into the correct spaces in a PDF (some internal knowledge of how OG creates a PDF from the drawing would be important). You can contact them using the Help -> Contact Omni with a test document so they can do some research from their end.
I tried going to multiple web sites that said “How to make you PDF ADA compliant” and most said “use the tools in the program!” Not helpful.