A recent update to the public API includes writable fields for the following resources:
created_at- a writable attribute for the people API that can be used to bring over the creation date of a record from another system
capital_in_cents- a writable attribute for the contact API to assign political or social capital, in cents
meetup_id- a writable attribute for the person API based on a person's unique ID in Meetup
created_at in particular should be used carefully as this is the field used by people to determine when a persons record was created in their database. In general it should not be updated.