MarketPress: Use order numbers from separate DB table

I'm wanting to know it's it's possible to modify the MarketPress order process so that it uses order numbers listed in a separate database table? The reason is this:

Here is Finland our banking system requires a special reference number when paying bills/invoices through online banking. The reference number can be created from various sources, but usually from the invoice number or invoice number and customer account number AND a check digit that is calculated using a special formula of the other numbers. This ensures that the reference number isn't input incorrectly. If the reference number isn't used, the bank charges more for a transfer.

Due to the unacceptably high cost of an automated online banking process (using, for example, Suomen Verkkomaksut), we cannot afford this through our small webshop and need to use a manual payment process in MarketPress. What we want is to have a list of these banking reference numbers and use them as the order numbers in the webshop. This would allow us to include all of the payment information in the manual payment email confirmation instead of having to send an additional email with a reference number and account details. The reference numbers would be non-consecutive and fairly unpredictable.

Although I am quite confident at PHP editing, I am not a programmer and cannot code the changes required to achieve this myself.

So, instead of the randomly generated order number, MarketPress would retrieve the next unused reference number from a table. I hope I've explained clearly.