Creating a custom formulae in BI Publisher Word template

Hi

Please can someone assist on how to create a custom formulae in BI Publisher word template. I read in user guide that this can be done but no where in my word template do I see where I can create this custom formulae. I want to calculate the EXTENDED_AMOUNT by the EXCHANGE_RATE on an AR invoice line and then insert the result as a secondary foreign currency column on the invoice.

Thank you