Help needed for getting firmware from board with AVR Dude

We are working on a project on behalf of a customer in which we are to integrate an existing, built-in Arduino board into a new board. Unfortunately, the original project is poorly documented. There are several variants of the firmware, but none seem to behave in the same way as the original firmware. Now we need help transferring the code from the existing board to our new board. We've already found out in forums that this works with AVR Dude, but we don't have time to familiarize ourselves with using it. We urgently need someone to help us with this.