Edit: This no longer appears to work as of at least macOS 14 and Apple Silicon. I believe Bluetooth Explorer worked (if you already had it) for at least some functionality on macOS 13 on Intel. On my current M2 system, not a single bit of Bluetooth Explorer works correctly, so if you have it you can probably trash it.
I just went through this using this no-name adapter on OS X 10.10: http://www.amazon.ca/dp/B00E38N7QE/ref=pe_386430_30332290_TE_3p_dp_1
The easiest way to use the USB adapter is to reboot. If you hold down the Option key while clicking on the Bluetooth menu icon, the MAC address of the adapter should be different. You can also verify in System Profiler. You'll know it's working if any existing pairings break.
I've found that a BT 4.0 adapter is working better than the built-in adapter in my 2010-era Macbook Pro. Especially with a headset, there's much better sound quality.
The on / off button in System Preferences is a little buggy. Turning off the adapter works fine, but clicking "on" turned the internal adapter back on. I had to unplug the adapter, plug it back in, and then turn Bluetooth back on.
If you want to work around this without rebooting, you can use Bluetooth Explorer from the OS X developer tools. You'll need a (free) developer account. There's a ton of useful tools to debug interference, pairing issues, and so on.

This is now part of a package called "Hardware IO Tools for Xcode V x.x" and can be found at https://developer.apple.com/downloads/