How can I propagate an "flexibility" parameter value into Windchill?

Hi,

A 3D model parameter PART_NUM is set with relation such as: PART_NUM = “SLMR ” + itos(d1)

The model is assembled in an assembly with flexibility to varies the length.

The Windchill product structure is report the default PART_NUM's value as set in the 3D model, but not the flexibility component's value

as set in the 3D assembly. 
 
How can I propagate an existing parameter value (with flexibility set to it), to be reflect in the Windchill product structure report?

Thank in advance.

Eileen