I can’t figure out how to delete this post but I figured out the answer. Segments are dynamic lists based on properties - so you can’t add someone to a segment explicitly but you can change the properties. 

@Fruition Seeds As you mentioned, to add someone to a Segment, you just need to know what conditions profiles need to meet to be automatically added to the Segment. If you can make those conditions true with an API request, that should add the profile to the Segment.