Ticket manipulation (similar to renumbering)

Hello, everyone!

I have an RT installation (RT 3.4.3, FedoraCore4, MySQL4.0.1) that has
recently been declared production. Unfortunately management declared it
production before I was ready, as there are approx 350 legacy issues I
wanted to load into the system before the newer tickets were created.

I need to be able to load those 350 legacy tickets (exported from a kludgy
custom tracking system) into the DB, with the recently created tickets
loaded afterwards. Ideally, I’d like to have the newer tickets begin with
a 500+ ticket number, but my pitiful attempts to make those changes
manually were ahem NOT successful.

I didn’t find anything in the wiki or in the book (I like the book, by the
way) that addressed this issue, nor did I find anything that I was able to
munge into performing the task. Can anyone point me to a process or howto?