you need to restore the seed to the nano... when the nano asks if you want to configure as a new device, select the button marked with an "X":
You will then be able to enter the KeepKey seed into the Nano which should effectively "clone" your KeepKey wallet into your Nano... you THEN use Electrum and select "use a hardware device"... it will detect your Nano and create a "watching only" wallet from the Nano's master public key.
This will ensure your seed is not exposed to any computer... it is completely entered (in a very long and tedious, but secure process) on the Nano itself.
NOTE: if you're unsure how to "reset" the Nano to get the "Configure as a new device?" message, (make sure you already have it's current seed safely written down if there are any coins on it
) and then simply insert an incorrect PIN 3 times...