How can we improve WHMCS?

Can we add the Registration Date and Next Due Dates fields to the AddOrder API, I need to import all my existing customers into WHMCS from a spreadsheet, I am doing this through the API which is great but as the orders are not NEW exactly, i need to set the start and end dates. I was advised I can do this using UpdateClientProduct but that will only let me update the nextduedate.