If we suspect that the WiFi network has been hacked and someone is using it to access sites, download files, etc can the origin of access be tracked or traced?
If you know the MAC address, then you should be able to locate the device rather easily. Some commercial tools that are useful for this are AirMagnet's Handheld Analyzer and Berkely Varitronics' Beetle B/A/N/G. There are probably some F/OSS solutions available for smartphones and/or laptops as well.
Once you've found a hardware/software solution that works for you, it's just a matter of playing "hot or cold" to hunt down the offending device by signal strength. A direction-finding antenna can help with this, but is not absolutely necessary.