Simply create a new bound object passing in the ODE placeable (a Body or GeomPlaceable) and a DisplayPlaceable from.
The same applies to XODE. Parse your XODE file (make sure every node has a unique name!) then iterate though the objects, binding the ones you are interested in to your custom geometry.
I am writing some documentation on this.
Here's some example code:
gun = new BoundDisplayObject(new Xith3DDisplayObject(gunTG), (PlaceableGeom) space.getGeom(this.name + "_" + "gunGeom"));
boundObjects is my DisplayBin instance.
When I parse the XODE file, I prefix all nodes with the a name an a unique number. So "gunGeom" becomes "mymodel-1_gunGeom". That way I can clone several instances and keep unique names.
Hope that helps,