Few days ago I made myself usb host adapter and it's working great with my thrusmaster t-wireless 3in1 gamepad with fpsece.
If you wan't to build adapter yourself you can either try to buy 24 pin connector that ipaq uses or hack one of those 2 adapters that came with your ipaq (like I did). I used one with usb cable attached to it, and later regreted that I didn't use the other one as that cable is really getting in the way when using joypad. All you needfrom spare parts is usb extender cable from which you cut off male plug and solder 4 wires to 24 pin ipaq connector.
Here is pinout of ipaq connector:http://forum.brighthand.com/showpost.php?p=1654053&postcount=7
You will need pins 5, 6, 7 and 8.
You can find usb pinout here:http://en.wikipedia.org/wiki/Universal_Serial_Bus
After soldering cable I had to cut the piece of plastic from 24 pin connector so I can close it and glue it back.
I tested it with usb stick, usb full keyboard, usb numeric keyboard (which can also be good replacement for gamepad), usb mouse, usb (wired) gamepad and usb wireless gamepad. It's all working flawlessly. I read somewhere that mouse pointer doesn't work. I was testing it only in Windows Mobile 6.5 and it IS working. You can use it same way as your desktop mouse and keyboard.
To use usb gamepad you will need this driver:http://www.fpsece.net/forum2/viewtopic.php?f=13&t=423
Alternatively, you can buy usb host adapter from e-bay, but they are too expensive for so simple cable.