Contact table: managing a new field
There is no hook or api specifically for writing to a field added to the civicrm_contact table, as that is a bad unsupported hack to core. I'd suggest changing your code to use the support provided for custom fields. That way you can be more confident that a code review of the extension won't end up preventing it from being published on civicrm.org for auto-distribution. If you still want to proceed with this bad pattern, then you might want to consider writing SQL and submitting it via db_query.