Hi Rachael,
Here is the larged thread related to this mod:
http://forum.x-cart.com/showthread.php?t=36756
Regarding Extra Fields data, there's a few options here. The mod allows automatic price break filters and also picks up product option / variant data. For Extra Fields we install an option so you can choose whether the field is multi value or single value as default. Either way, each of these extra fields supports import and export.
Feel free to drop me a call if you want more details.
Regards,
Craig