How do I update a manually calculated tax amount to the Vertex register?

We are currently on E1 9.0 on the iSeries.  We are using Vertex but I'm not sure what version.  We use the Sales Order Management for most of our invoicing.  However, we have situations in our route distribution business where we generate a manual invoice that includes taxes.  We need to import that manual tax amount in the Vertex register.  Does anyone know if there is a business function or application that we can use to write these tax amounts into the Vertex register when we import these sales?

Thank you.