Hello,
i guess, while talking about "part number", you mean PARTNER_PROD field in Shopping Cart (Supplier product number).
As this number is the product reference from the supplier point of view, it can only exits when purchasing from a catalog (Punch-Out or internal).
So, this value cannot exist when you create a Free-Text SC item ("Can't find" tab in SRM UI Add-On)...
For Free-Text, the only reference is the product category.
Regards.
Laurent.