Center of Gravity of part in a assembly reference coordinate system

Hello 

I am faily new to creo 

i am using creo 7

I have an assembly and have defined a custom coordinate system origin in the assembly file

i am trying to get the center of gravity of a part in the assembly with respect to my "assembly" custom CS

What i am seeing is:

1) i can only open the part and get the CS of the part with respect to a CS "inside" the part file.

or

2) i can get the CG of the whole assembly with respect to a custom CS.

both options are not what i need

i need the CG of a part in an assembly with respect to a custom "assembly" CS

any help would be greatly appreciated. 

thanks