Ability to load fixed VAT and or TAX amount in Billing Interface and bypass PS defaulting

We are wondering if anyone has any experience with loading the Billing Interface from outside systems.

In our case we are getting daily transactions from a source system maintained by one of our partners/suppliers mainly for B2C activities. The bills we want to load can have VAT or TAX included, but our system does not have all the VAT or TAX rules defined, as we do not need these rules for our B2B activities and we are not actually billing the End Customer, but our partner/supplier.

Is there a way we can force the VAT and or TAX amount in the billing interface so it won't be overwritten by the VAT defaults processing as we need to exactly follow what our partner/supplier has invoiced the end customer.