How Can I Add a Computed Field Value to a Database Query Output?

I have a database row like this:

invoice
   description
   amount_invoiced

When I do a multiple record query on this database table, I’d like to also output an extra field I compute at the time of output:

invoice
   description
   amount_invoiced
   amount_accounting_currency (a_set_value_value * amount_invoiced)

How can I do this in a Server Action?