Support can add these for you. Be sure to include the Display Name, API name, and Type(Date and Datetime are separate) for each field you're requesting.
The API you're referring to is only for creating and updating custom object records, we do not currently offer an API for managing custom object types.
Sorry if I was unclear, but I'd like to see some of the example requests and responses where you're observing this behavior, rather than just a description.