To the best of my knowledge, Crystal does not natively support exporting to .dat files.  This is primarily because .dat files are, at their lowest level, just generic CSV files.  But, they are generally then very specifically formatted for the purpose they are intended (q.v., winmail.dat, which can only be usefully read by Outlook).  You may be able to simply use a CSV export, and then rename the extension afterwards.  You may be able to find a third-party product that already does what you need (check out Ken Hamady's excellent list).  But, I'd be willing to wager that you won't have an "out of the box" solution for this.