incrementally appending custom XMP metadata into PDF files

I have a problem writing custom XMP metadata into a PDF document (I'm using the XMP Toolkit SDK).

The problem is that I don't know whether is possible to expand the XMP packet in order to append metadata incrementally into the document, and, if it is possible, I don't know how to do it. The program I wrote succesfully appends an item to an array four times, but it fails when trying to append the fifth one. The reason obviously is that the packet gets full and there is not room for more metadata. The error message, if I force the PutXMP call, is:

ERROR: Can't fit into specified packet size

So, what can I do? How to enlarge the packet or make it expandable? Or, otherwise, have I to inject a new XMP packet with the up-to-date information in it? How?

And a last question: Have I to scan for packets when dealing with PDF files? No smart handlers are available for this kind of files?

Thanks in advance.

Xabier Artola.
Univ. of the Basque Country.