Only if the PDA supports being a USB *master*. Most devices do NOT; they are USB *slave* devices and can NOT control another USB device (such as a USB->serial).
There is a new standard for USB devices that allows a device to be both slave (for sync to a PC) and master (to use USB accessories). I'm not aware of a PDA that has that type of USB port.
Note that this is a *hardware* issue - the device either has master-capable hardware or it does not. If it does not, there is nothing you can do to change it.
Some PDA's have a serial link as well as a USB connection (my IPAQ, for example). If your PDA has serial capability, it can be used directly with the proper cable (and some terminal software).
John