Programatically creating users using external authentication backends

I was wondering if it was possible to create users which would authenticate against LDAP (or other external authentication provider) via the REST API (so that users could be set up without requiring the user to have logged in yet)? From what I can see on, the REST API can only create local users, not external ones. Would this be a feature that could be added?

I don’t think so. LDAP is used as a read-only source. This is true for pretty much all the external user providers.
You’ll have to do it on your own, not through ownCloud. You can create a script to add the user to your LDAP server and then sync the user in ownCloud (assuming you have the connection between ownCloud and the LDAP server correctly configured)