I did some search here in the forum and did only find unmaintained third party importers for Excel CSV. And no other workaround.
In my opinion an importer for Members via Excel CSV(or numbers or opendoc) should be CORE. Why? It is a helper for admins that use XF for the first time (like me) and have to import members from another source – other than other brand's boards... (like I do: import from a professional member administration software for my club).
This is a marketing tool and great selling argument for XF core, that no matter how you come to XF, you will be served.
Think about it. I am shortly before completion of my forum after one year of work on it. Looking forward to import 1000+ members by hand lets me shiver.