The shuttle may or may not be user-installable (still not clear), but the kitchen modules themselves are designed to be removed/installed by the user.After market implies a manufacturer other than Rivian. So to answer that question: Yes, there are other companies that make camp kitchens for the back of trucks/suvs. However, it will be different than the Rivian unit and wouldn't slide in/out of the gear tunnel.
If you really meant to ask "can the OEM camp kitchen be added after initial delivery" then I believe the answer is also "yes", although it would most likely require a visit with Rivian service for installation. Rivian has previously said that DIY installation was unlikely to be supported.