To force your system to use an external Bluetooth controller instead of the internal one, open Terminal and type:
sudo nvram bluetoothHostControllerSwitchBehavior=always
then reboot. To revert to the default OS X behavior, type:
sudo nvram -d bluetoothHostControllerSwitchBehavior
You may need to turn off (or un-pair) Bluetooth accessories that are currently using your internal Bluetooth controller, as they can prevent the dongle from being detected by the Continuity Activation Tool.