Tax Calculation in SAP Purchase Order

Hi Experts,

My Requirment is so get the US and Canadian tax automatically from external tax system ( other than Vertax Sabrix and taxware) while creating Purchase Order. We are using SAP XI to get the TAX AMOUNT from external system.

The probelm is , the external system provides me directly tax amount rather than tax code. As per my knowledge , the tax calculation in Sap is done through tax code only.

Please help me on this. Do I need to do something like , create a new condition type and put the tax amount there ?