Adding XMPMeta with PDFBox - pdfbox

Question

I try to implement XMP using PDFBOX Library.
I try to output the following XML data:
<rdf:Description xmlns:pdfaExtension="http://www.aiim.org/pdfa/ns/extension/" xmlns:pdfaSchema="http://www.aiim.org/pdfa/ns/schema#" xmlns:pdfaProperty="http://www.aiim.org/pdfa/ns/property#" rdf:about="">
<pdfaExtension:schemas>
<rdf:Bag>
<rdf:li rdf:parseType="Resource">
<pdfaSchema:schema>Factur-X PDFA Extension Schema</pdfaSchema:schema>
<pdfaSchema:namespaceURI>urn:factur-x:pdfa:CrossIndustryDocument:invoice:1p0#</pdfaSchema:namespaceURI>
<pdfaSchema:prefix>fx</pdfaSchema:prefix>
<pdfaSchema:property>
<rdf:Seq>
<rdf:li rdf:parseType="Resource">
<pdfaProperty:name>DocumentFileName</pdfaProperty:name>
<pdfaProperty:valueType>Text</pdfaProperty:valueType>
<pdfaProperty:category>external</pdfaProperty:category>
<pdfaProperty:description>name of the embedded XML invoice file</pdfaProperty:description>
</rdf:li>
...
<rdf:li rdf:parseType="Resource">
<pdfaProperty:name>ConformanceLevel</pdfaProperty:name>
<pdfaProperty:valueType>Text</pdfaProperty:valueType>
<pdfaProperty:category>external</pdfaProperty:category>
<pdfaProperty:description>The conformance level of the embedded Factur-X data</pdfaProperty:description>
</rdf:li>
</rdf:Seq>
</pdfaSchema:property>
</rdf:li>
</rdf:Bag>
</pdfaExtension:schemas>
</rdf:Description>
For this I used a XMPSchema object to build it :
XMPSchema pdfaExtension = xmp.createAndAddXMPBasicSchema();
pdfaExtension.setNamespace("http://www.aiim.org/pdfa/ns/extension/");
pdfaExtension.setPrefix("pdfaExtension");
pdfaExtension.addBagValueAsSimple("schemas", "schemas");
This allow me to generate the following:
<pdfaExtension:schemas>
<rdf:Bag>
<rdf:li>schemas</rdf:li>
</rdf:Bag>
</pdfaExtension:schemas>
My issue is to create "nodes" under the rdf:li tag.
I tried using the following methods :
pdfaExtension.addBagValueAsSimple("schemas", "schemas");
pdfaExtension.addQualifiedBagValue("BagContainer", "Value3");
pdfaExtension.addUnqualifiedSequenceValue("SeqContainer", "Value1");
pdfaExtension.addProperty(xmp.getTypeMapping()
.createText(null, "test", "simpleProperty", "Hello"));
Unfortunatelly, I do not retrieve the correct output.