Combine two RDF datasets

Hi,

Is there any way to combine two RDF datasets? I have one XML-file
dataset which I load at startup. I want to add additional triples
during runtime. The easiest way for me to create these triples is
using a string, e.g.,

==================================
var strRDFData1 = '\
<?xml version="1.0" encoding="utf-8"?>\
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"\
         xmlns:email="abc://my/car#"\
         xmlns:attachment="abc://my/part#">\
<rdf:Description>\
  <car:model>Chevy</car:model>\
  <car:parts>\
    <rdf:Bag>\
      <rdf:li>\
        <rdf:Description>\
          <part:name>Some part name</part:name >\
          <part:size>12</part:size>\
        </rdf:Description>\
      </rdf:li>\
    </rdf:Bag>\
  </car:parts>\
</rdf:Description>\
</rdf:RDF>';
==================================

Then parse the string into a in memory data set. However,  I see no
way how to combine such in memory DS with the XML-file data set (which
is composed of triples like the above).

If I try to use the current XML-file data set as the destination of
the parser, the current data in the XML-file data set is __erased__
before the data in the in-memory dataset is appended. I do not want
such a thing. Could someone please help?

Thanks