Anyone know of a way to add a Homebrew Invocation to the Warlock class?

Create it as a feat and just select nothing from the invocation dropdown.

The other alternative is to create a 2nd level feature attached to a homebrew warlock subclass named “Eldritch Invocations,” and essentially recreate the original class feature including a separate Option to recreate each and every one of the existing official Invocations from scratch in addition to the Options you would add for your homebrew invocations. That would cause the subclass feature to overwrite the base class feature. But you would have to do it for each of the warlock subclasses individually too.

Thanks interesting idea. I could do that and add the Invocations my character wants.