search based on invoice number in document manager to get documents of declaration & order release

I would to like perform a search based on invoice number in document manager so that I can get the documents of declaration and order release of that particular invoice all together at one single search. I was not able to add the UDA(as i dont see that option at all)/ flex fields on document (restricted for document table) / Document reference qualifiers.

Please suggest how can i add the reference qualifier (invoice number) from declaration to document so that my search can be done