I have found a workaround for this problem by my own.
I have added a response handler to the webservice that add a dummy attachment part to the soap message and activate mtom using the method org.jboss.ws.extensions.xop.XOPContext#setMTOMEnabled(true) and declare the message as a XOP Message. That's all.
Code Sample:
public boolean handleMessage( MessageContext msgContext ) {
....
... // decide if outbound and other code left out....
....
org.jboss.ws.core.soap.SOAPMessage msg = ((SOAPMessageContext) msgContext).getMessage();
org.jboss.ws.core.CommonMessageContext cmCtx = (org.jboss.ws.core.CommonMessageContext) msgContext;
ByteArrayDataSource DUMMY_PLAIN_DATA_SOURCE;
DUMMY_PLAIN_DATA_SOURCE = new ByteArrayDataSource( "DUMMY_ATTACHMENT_TO_FORCE_MTOM_RESPONSE", "text/plain" );
msg.addAttachmentPart( msg.createAttachmentPart( new DataHandler( DUMMY_PLAIN_DATA_SOURCE ) ) );
((SOAPMessageImpl) msg).setXOPMessage( true );
Scope lastScope = cmCtx.getCurrentScope();
cmCtx.setCurrentScope( Scope.APPLICATION );
XOPContext.setMTOMEnabled( true );
cmCtx.setCurrentScope( lastScope );
...
}