Overhead Absorption for custom Transaction Type Sources

 We are successfully getting Overhead Absorption calculated for our PO Receipt transactions.  However, when we perform a receipt fo rthe same items and org using a custom Transaction Type assigned to a custom Transaction Source, we do not get Overhead Absorption calculated.  Is the calculation of Overhead Absorption limited to specific seeded Transaction Types and Transaction Type Sources?  Can this be extended to calculate for custom Transaction Types and custom Transaction Type Sources?  If so, how.  Thanks...Todd