How do I design pcb keypad contacts for hard gold plating
Just make an additional Gerber layer that exposes just the button contact regions.
Next you describe that layer in your fab drawing and fab notes documentation what that layer is for along with all your other layers.
Almost all PCB fab houses can take it from there it adapt to their hard gold process. That may mean they invert the supplied Gerber in terms of negative or positive image and over or under size the features as needed.
By the way...your comment about not having a fab house to talk to is silly. Pick out one or two and contact them for more specific details about how you can make the Gerber layer most useful to them for their process.