E1 8.12 Brazil Localizations - ISS calculations on non PO vouchers

We currently only have Brazil localizations setup only for Withholdings on Service type vouchers and payments.  Is there a way to calculate different ISS rates on a stand alone voucher (non PO)? Usually on a stand alone voucher, the R76B4310 calculates the ISS based on either a) Supplier Master default values or b) Procession Option overrides.  But the calculation is based on the entire voucher gross amount. For example: on a 10,000 voucher, 6,000 should be at the 2% service w/h rate, but the remaining 4,000 should be exempt.  How can we do this without using the Procurement module which