If you get a gwt object from a jsni method, you could pass it to another jsni method, but you can not manipulate it in js unless you know the gwt internals of this object.
This should work, if not there is a bug in gwt-exporter
var o = mynamespace.getMyGwtInstance();
mynamespace.myMethod(o);
public MyObject getMyGwtInstance() {
return new MyObject(....);
}
public void myMedhod(MyObject gwtObject) {