Can you overwrite the tax amount in A/R and A/P but retain the original tax rate/area and tax explan

We are trying to integrate a subsidiary company's (Canada) financial transactions into our JDE system, which includes A/R , A/P and journal entries.  For A/R, their calculation of tax is not as simple as Canadian province/area, but several complicated factors.  We are looking uploading invoices into our system using the invoice batch upload program, and have their customer master already entered in our system with default tax rate/area based on the customer address/province.  But we'll need to be able to overwrite the tax amount based on several other factors.  The invoices are not going to go through the Contract Billing/Service