FogBugz ships with a script for importing bugs from Bugzilla into FogBugz. This file is called importBugzilla.asp on Windows systems and importBugzilla.php on Unix and Mac systems. To get started, go to:
(Windows FogBugz server) http://fogbugz URL/importBugzilla.asp
(Unix/Mac FogBugz server) http://fogbugz URL/importBugzilla.php