How to force Mac (Yosemite) to use USB Bluetooth instead of internal?
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.