I am uncertain what you are doing--how you are preparing for exporting to XML. I am also uncertain what you are doing for import. If I tag everything for export, including character styles, then upon import I can map those tags to styles. Is this what you are doing?
Perhaps it would be good to know what you are wanting to accomplish for exporting as XML and its reimport.
Please note I may be off-line for most/all the day so responding by me might be delayed.
Take care, Mike