Is a copy of JIRA going to be kept around somewhere, ideally web-readable (static HTML?) or just as database backup? I don't understand if you're only speaking of open or also closed tickets. Would you want to first do a triaging on JIRA to see what's still current and migrate the rest, or migrate everything? I can't help with scripts and I think all the designated tickets must be "dumped" into bugzilla automatically for later triaging; I can help with the triaging at whatever stage, let me know.
Nemo