DOMToDoc function - take a specific node from Process Data

Request
Take UM element from Process Data and put it into PrimaryDocument with DOMToDoc() function.
Process Data
<?xml version="1.0" encoding="UTF-8"?>
<ProcessData>
<PrimaryDocument SCIObjectID="MIRJANA:node1:1316a763640:145462"/>
<UM>
<T1>aaa</T1>
<T2>bbb</T2>
</UM>
<Other>ggg</Other>
</ProcessData>
Primary Document after the operation should be:
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<UM>
<T1>aaa</T1>
<T2>bbb</T2>
</UM>
1. DOMToDoc function
DOMToDoc function setting
<assign to="." from="DOMToDoc(//*[name() = 'UM'], 'PrimaryDocument', 'no', 'UM')"></assign>
… or even more simple …
<assign to="." from="DOMToDoc(//UM,'PrimaryDocument','no', 'UM')"></assign>
2. XML Encoder
<operation name="XML Encoder">
<participant name="XMLEncoder"/>
<output message="XMLEncoderTypeInputMessage">
<assign to="." from="*"/>
<assign to="mode">process_data_to_document</assign>
<assign to="root_element">UM</assign>
<assign to="xPath">//UM/*</assign>
</output>
<input message="inmsg">
<assign to="." from="*"/>
</input>
</operation>