Hi Andy,
the fuctionality of order combination (IS-MILL) is not released for SAP APO (http://service.sap.com/sap/support/notes/1406424). Please check SAP note 1406424 for more details http://service.sap.com/sap/support/notes/1820289.
The "Interface for Industry-Specific Optimizer" of SAP APO can be used to connect external trim optimization tools: http://help.sap.com/saphelp_apo700_ehp03_on_erp/helpdata/en/c3/4fc95360267614e10000000a174cb4/frameset.htm. This functionallity is not based on the order combination in IS-MILL.
Best regards
Kai