The flexible keyboards I see on Adesso's website are for PCs, and I don't think they'd work, even if you could find cable adapters to mate standard USB to the iPAQs port. You'd need a keyboard driver too, and they don't offer any for PDAs.
There are keyboards made for PDAs, that connect via Bluetooth or IR, and these come with drivers as needed.
Here are reviews of some on the market: http://www.mobiletechreview.com/tips/keyboards.htm . The reviews near the top of the page cover more recent models that should still be on the market. The Stowaway keyboard products we reviewed are now sold under the iGo brand rather than ThinkOutside.