You should take a look at the xmp2iptc.args file included in the
full distribution if you want to copy XMP information to IPTC.
In this file, it gives an example command line to copy XMP information
from one JPEG image to another, but the command is the same for
copying from a .XMP file to a .JPG.
You can also copy XMP to XMP by adding a "-xmp:all" to the command
line after the -tagsfromfile option.
I hope this helps. I'm not really clear about exactly what you wanted
to do.
- Phil