India GST

Our business is doing lot of services and we can't able to create an item for each services. We will be go with as per current practise to capture the description while create a Requisition/PO.

We will be planning to use SAC code as an internal item and it will be captured in 'Non Catalogue' (iProcurement) request as additional information. Since the requisition/Purchase order don't have a item the tax engine is not defaulting the Tax Category. Is there any way we can add our additional condition/custom logic into the tax engine to derive the tax default?.