Perhaps you can try this as well.
Goto
Tools->Infrastructure->General
Select Keep link with selected object
1. Open part
2. create a parameter "Param_1"
3. open product
4. create a parameter "Param_2"
5. goto formula definition for "Param_2"
7. once opened select the "Param_1" of part.
External parameter will be created along with formula.
Regards,
Maddy