This feature is since XFRX 18.1

* create PDFL#INIT object
loSession=EVALUATE([xfrx("PDFL#INIT")])
* open existing PDF document with append mode
loSession.createDocument("BASE-UNI.pdf", .T.) 
* Add attachment - PDF file
loSession.addAttachment("some.pdf",.T.,"","appliaction/pdf","Alternative")
* Add attachment - some XML file
loSession.addAttachment("some.xml",.T.,"","text/xml","Alternative") 
* Finish document
loSession.FinishDocument()