The short answer is no. You cannot add new fields to the program.

You did not indicate what functionality the information in this new field would provide so I can only guess at a couple of workarounds for you.

If you wish to use the account number for selection of the customer during transactions and allow for searching then create your account number as part of the customer name. For example, you may have a customer called Heavy Lifting Inc. and you use the first three characters of each name as the account number so you would use HEALIF - Heavy Lifting Inc. as the customer name (only works when customer names are less than about 43 characters).

If you only need to search by customer account number and not select by it then you can use the Contact field. That will allow you to search for any invoice, receipt or the ledger for the address itself.

If all you need to do is display it on the printed invoice, then you can use one of the additional info fields on each of the ledgers, then modify the template to show that field.

I used customers as an example, but the same idea applies to vendors and employees as well.