PDF files not supported by XMP SDK

[Christoph Dalitz]

Dear all,

just realized that my problem of adding XMP data to a PDF file
cannot be achieved with libexempi, because the underlying XMP SDK
by Adobe does ironically not support PDF files as stated on Adobe's
site:

http://support.adobe.com/devsup/devsup.nsf/docs/52016.htm

Thus I conclude, the only way is to add a generously padded empty 
XMP data stream with some other software and edit it afterwards with
exempi.

Does anyone know of a free software that can add a data stream to
a PDF file?

Or is there even a way to specify the XMP data in a Postscript file
(which is the original file that I am writing with my own software)?
This would perhaps let me avoid the entire PDF mess.

Thanks,

Christoph