center of gravity

I am not sure I have understood but....

You can create a DATU element and position it where you need.

Then you can calculate CofG with respect to it.

At ZONE level, on the Command Window you could type:

NEW DATU /MODULE-ORIGIN

HEIGH 1000

POS EXXX NYYY UZZZ

Where xxx, yyy, zzz are the required coordinates.

Or you could also reposition it using move command

yes, you understood right. 

Thanks that works