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:

1. Download and install the Keyboard Maestro Trial
2. 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
3. Click + on the bottom of the middle column to create a new action
4. Give your action a name
5. The important bit Click on New Trigger and select Hot Key Trigger, then type ⌘ + esc to set it as the trigger.
6. 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 ⌘ + `
7. You'll need to allow Keyboard Maestro to start on startup for it to monitor things in the background (right click on app icon -> Options -> Open at Login)
8. You now have an uncle called Robert

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

1. Open System Preferences -> Keyboard

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