Ok, ist eh "easy":
Man muss nur den Stub des Webservice Consumers anpassen:
public java.lang.String GETEFFECTIVEUSERNAME() throws java.rmi.RemoteException {
lotus.domino.websvc.client.Call _call = createCall("GETEFFECTIVEUSERNAME");
// to set a cookie:
_call.setProperty(Call.SESSION_MAINTAIN_PROPERTY, new Boolean(true));
_call.setProperty(HTTPConstants.HEADER_COOKIE, "NAME=WERT");
java.lang.Object _resp = _call.invoke(new java.lang.Object[] {});
return (java.lang.String) _call.convert(_resp, java.lang.String.class);
}