Toggle Bluetooth using wired keyboard on iMac
Nimesh’s answer is very clever and requires no extra tools, so that may be what you want.

However, there is a much easier way, if you are willing to spend a few minutes to download and install some apps/tools once in order to have a simpler way to turn Bluetooth off/on in the future.

First, download and install blueutil from http://www.frederikseiffert.de/blueutil/download/blueutil.zip

Unzip it, double-click the Install.command and enter your password, and it will be installed to /usr/local/bin/blueutil. 

(If you use brew, ignore the above and just do: brew install blueutil) 

After that, you just need a way to run it via the keyboard. For that, I highly recommend Alfred, although you could use LaunchBar instead.

Assuming you use Alfred, get this Bluetooth toggle workflow and you can enable/disable Bluetooth with a few keystrokes.