APO GATP Location replacement

Hi Gurus,

In APO GATP, location substitution works by first checking the stock in the base location (the location from where the material availability check happens) and if stock is not available in the base location only then it cascades/ substitutes to another location. But, we have a scenario wherein when material availability check happens it should straightaway carry the location substitution without even checking the availability at the base location; the availability check & delivery should only happen from the substituted location; backorder (open quantity) also sits at the substituted location.

Can you please give me the various customizing options to map the above requirement in SAP SCM?