It's a longstanding issue indeed, and annoying as @%@#$ I also love to use the DateAdded as a sort criterion.
I'm the developer of the DJ Conversion Utility and maintain excellent relations with the Pioneer engineering team. They are aware of this problem, as well as the build up of import times of XMLs when the Rekordbox collection grows.
So we have to sit & wait for them to fix it.
It's been a while I dived into it but I think I found out (not 100% sure) that the setting of the date format in the System preferences influences how the DateAdded is being imported but I'm not 100%, couldn't get it working reliably anyway :(