assign expressions to an existing wtpart structure

I have a wtpart structure created by a cad structure and want to assign an expression from an external source. (preferably xml file) to it.

I found the ITO loader method for loading whole project for options and variant’s. but when I use this method, Windchill creates a new structure in the existing end item. And is not using the structure created by Creo.

I used the sample from the windchill dir <Windchill>\loadFiles\ato\demodata as a guide.

Does anyone know how I can assign expression to the existing structure without adding wtparts?