How to migrate withholding tax in the purchase order

Hi ,

One of our customer in going live in next couple of month, I want to know how the data related withholding tax will be migrate from legacy to AX 2012.

In case of purchase order for service type item withholding tax is applicable in UK. But the tax calculation will be done at the time of making payment to the supplier. So want to know how to migrate the data.

rgds

dhananjay