Unless I'm missing something, it appears that the Additional Name Fields feature of 2.6 isn't supported in the User Totara Sync currently. Without the ability to update the profile fields (alternate name, middle name, etc.) using Totara Sync, the feature isn't actually very helpful because there's no way that we would want to manually update 1000s of users. Hopefully this is something simple enough that it can be included in a very early point release of the 2.6 series. I am aware from MDL-40683: CSV user uploads do not allow alternate name fields that there is a workaround, but Totara Sync is definitely the better tool to use for user management.
Regards
Dan