PPM-Projects Contract-Is there any way to use 2 different Exchanged Rates

Summary:

Is there any option to use 2 different exchange rates on the PPM Contracts

Content (required):

Hello

In Mexico City we have several USD Contracts and we usually use the Central Bank Exchange Rate, however for some invoices is required to use another exchange rate i.e. user defined.

Hence I was wondering for those 5 monthly invoices exceptions if we can do an amend on the contract on the currency conversion tab, I will change the exchange rate from Central Bank to user defined and once the invoice is generated I will reverted it back from user-defined to Central Bank.