I think you may be out of luck. The Pro Duo specification came out years after the AV100, so it is probable that the reader is not compatible, and Archos hardly ever make firmware updates after a generation has been replaced - assuming that the update is even possible in software, it may need a new hardware reader.
The only thing I could think of as a solution is a ProDuo to Compact Flash adaptor like this http://www.amazon.co.uk/Ex-Pro-Compact- ... B000XEJJFU
plugged into the Archos CF reader. I used the replacement SDHC type in the past with SDHC cards and it did work (slowly!). Unfortunately there is no demand anymore for this type of thing so it may be hard to find - Ebay?
Alternatively a Pro Duo to ordinary memory stick adaptor plugged into the Archos adaptor if you can find such a thing?