Define Inventory Org which has a Costing Org different than itself

1)  Defined Inventory Org ABC, it's Costing Org is ABC

2)  Defined Inventory Org DEF, I want it's Costing Org to be ABC.  The Costing Org field is defaulted as DEF and cannot be changed to ABC

3)  I believe you are able to have a Costing Org different from the Inventory Org - how is this done?

4)  In both of the above Inventory Orgs, the Item Master Org is the same, e.g. XYZ  We have approx 20 other Inventory Orgs which are their own Costing Orgs.

Thanks in Advance