How do I transfer a file from one BlackBerry device to another over WiFi like I've done with blue tooth?
In Bluetooth, each device can easily become aware of one another because the protocol supports this. In Wifi (which is just a medium for generic networking... TCP/IP in many cases), it's generally expected that one machine already knows how to locate the other... so this is the problem you need to solve.
One option is that you can have one of the devices (or even both) periodically broadcast a message when it wants to connect to something; this message would be on a pre-defined port but as a broadcast, it's open to all receivers. Then the other device (or even both) needs to have a broadcast receiver looking for the message on the right port. One benefit here is the broadcast receiver will not only receive the message, it will also receive the IP address of the sender -- this is your missing component.
Once the receiver has the IP address of the sender, it needs to open a connection to the server port running on the device that sent out the broadcast. Of course, that first device needs to have its server task running at this time also.