Unfortunately, no. WiFi works on a completely different frequency and modulation than cellular. You would need some kind of cellular access device (like a Sierra AirCard, or a cellphone and data cable) to get onto any cellular network.
Alternatively, you can get a bluetooth-enabled cellphone (Ericcson makes one, as I recall) and link to it via your bluetooth connection on the iPAQ. The cellphone would act as a modem for your PDA. This way, you don't have to deal with a cable; you can leave the phone in your pocket and hold the iPAQ.
Cheers,
-Pat