How to add keyboard shortcut for 'move focus to next window'
It can be done!
Option 1: Create a new shortcut using Keyboard Maestro
NOTE:

You can do what I've outlined here within the free trial of the app I believe, but I can't guarantee that'll always be the case.
I am not affiliated with Keyboard Maestro or advertising for them, I just happen to know that you can create system shortcuts using their app.

Steps:

Download and install the Keyboard Maestro Trial
Once you open the app an editor window will appear. On the left you'll see folders, in the central column you'll see a bunch of actions, and if you click on one you'll see the configuration options for each action
Click + on the bottom of the middle column to create a new action

Give your action a name
The important bit Click on New Trigger and select Hot Key Trigger, then type ⌘ + esc to set it as the trigger.
The other important bit Click on the box the says 'No Action', this brings up an action picker on the left

Type 'Keystroke' into the search box
Drag the action called 'Type a keystroke' into the actions box
Click where it says 'Simulate Keystroke' and tap ⌘ + `



You'll need to allow Keyboard Maestro to start on startup for it to monitor things in the background (right click on app icon -&gt; Options -&gt; Open at Login)
You now have an uncle called Robert

Option 2: Change the System Shortcut (This doesn't add a secondary sadly)

Open System Preferences -&gt; Keyboard



Then select the Shortcuts tab at the top
Select Keyboard in the panel on the left
Click on the ⌘' symbol next to Move focus to next window
Type what you want it to be on your HHKB keyboard