I looked through some information from a few different vendors. I didn’t see anything that would allow new patients to be registered. Am I missing something or is that generally not allowed with FHIR?
Patient registration is typically a process tied into complex (and EHR-specific) workflows. It’s also not typically something delegated to external systems. As such, I’m not aware of any EHRs that have introduced this capability so far. (They’ve been working to get out easier functionality, such as read access to data and write access to “easy” data, such as Observations.) That doesn’t mean it won’t ever happen. You might raise the question on chat.fhir.org which tends to be more monitored by the EHR vendor community.
It really depends on the EHR, and what exactly you mean by “registered”. For example, Epic does support the Patient create interaction. In general most EHRs have started with read interactions and are now gradually adding write interactions.