Hi Panupong,
Could you post the code you wrote? Target XML should be empty message payload without a root element is generated. This could be done only by creating a Java mapping and implement the logic to convert from root element to an empty text. This can not be done using Message Mapping tool.
I did not try this with multimapping but sincerely hope it will work for multimapping too. Please check trhis thread for adapter module(StrictXml2PlainBean) based approach to your problem.
http://scn.sap.com/thread/3162134
Regards
Anupam