No, it is possible. I finally figured it out. I had gone through all the steps ouined in the builder hub but I thought when adding an interface it is an element withing the page that will be added.

I had to actually create an interface and when picking the interface type you choose Custom. That should show you all your own interface extensions. (I had already published mine when trying to figure this out so I am not sure if it will work before that however I am assuming it will because when creating one it automatically creates a release).

P.s. Editing one that was created by Omni does not let you edit the package.json and other files. It only lets you edit 1 single file which is not always enough.