I'm on an older Mac Pro, and I bought the ASUS USB-BT400 so that I can use Yosemite's Handoff feature. The problem is, I don't know how to disable the internal Bluetooth and force my machine to use the USB Bluetooth instead. I searched around but didn't find anything that worked.
Continuity Activation Tool is a great app for doing all of the command-line stuff necessary to enable Continuity, and it claims to be able to work with USB dongles.
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.