Sure, if you find someone that wants to design and manufacture a niche keyboard for a niche laptop, you could do that. All the ports are documented and I believe they’re simple USB2 with a couple additional pins.
I would love third-party add-ons for the Framework laptop but I don’t have a high confidence that that’ll happen. AFAIK it hasn’t happened with the Expansion cards either - Framework announced that 3rd-parties could sell their cards in the Framework marketplace, but I don’t see any except for the official ones, and I also can’t find anyone manufacturing custom modules on the internet either - except for some people making their own custom modules with a 3D printer.
And making such an expansion module would be way easier than making a custom keyboard, I assume. And the expansion modules could at least in theory be used on other devices as an awkwardly-shaped dongle. The keyboard would basically be useless for other laptops.