We aren't sure as to the "when" but the Windows Phone app/tool for Windows 8 was recently updated. The update adds the ability to transfer playlists and ringtones to your Windows Phone 8 device and watch videos you've taken with your Windows Phone.
Ringtone files must be
.m4r format any non-DRM protected music file that the phone can play, no larger than 30MB and can not be protected with digital rights management. When you transfer the ringtones to your Windows Phone 8 device, they will appear in your Settings under Ringtones and Sounds as any other tone would be.
If you haven't installed the Windows Phone app to your Windows 8 computer or tablet, it's a free app that you can find here in the Windows Store.
Thanks, Andy, for the tip!