Let's say you have WSDL parts defined as follow:
<wsdl:message name="HelloMessage"> <wsdl:part name="part1" type="xsd:string"/> <wsdl:part name="part2" type="xsd:string"/> </wsdl:message>
In order to build whole message at once, you need to use following statement:
<assign> <copy> <from><literal><message xmlns=""><part1>abc1</part1><part2>abc2</part2></message></literal></from> <to>$myVar</to> </copy> </assign>
Complete BPEL example is provided here: TestMultiPartMessage