External USB Keyboard to Android (via bluetooth preferably)
If it's an USB keyboard, it can be directly connected to the MicroUSB port of your Android device using an OTG cable. If your device is OTG capable (most devices of the current generation are, so chances are pretty good), it should be detected automatically. That's what I do with my tablets (running Android 4.1) and a tablet-case with integrated keyboard, to transform them into a mini-notebook – it works fantastic!

As a USB keyboard usually doesn't have Bluetooth integrated, the BT approach would require some kind of adapter. I won't say there is no such thing, but at least I've never heard of any.