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If not, then you're probably going to have to get a hex editor and deconstruct the file manually (unless the software is open source, or someone has already posted file structure info on the web somewhere - Google is your friend :)
My project was based on QuickBooks, and all I needed to do was generate invoices that matched the same accounting data as was in MySQL. After a couple of phone calls to Intuit, they gave me a url (which I had seen but ignored) which explained how to create a Tab delimited file which could be imported into QuickBooks.
Moral: Call Peachtree Software technical support to ask them if they have a solution for importing data into their database. If it has an import feature, then they should be able to tell you how to use it, right?
PS: Keep a copy of an origional invoice handy so you can compare the data you enter with the data that is normally entered by hand.