SPAIN - How do I add a VAT only invoice with customs tax?

Our Spanish entity would like the ability to enter a VAT
only invoice with customs tax in order to pay both the VAT amount and the
customs tax when an imported good clears customs.

Currently in PeopleSoft you can enter VAT only invoices by
utilizing third party vouchers, where you can enter only the VAT amount to be
paid and the VAT Basis amount on the voucher line. However, there isn't a field
on this voucher to enter the customs tax. We would like to be able to not only
enter the VAT to be paid but also the customs tax on the invoice. We would like