If you want to use iMovie I think that the answer is probably to get a new DV iMac.
A Firewire/USB converter is not on the cards. DV requires approx 5 MB/s throughput to capture and USB only provides a bit over 1 MB/s.
There is also the problem of software to support these things. For video capture over USB try the Interview from
http://www.xlr8.com.
Standard disclaimer there. This is what I think is the case, I'm happy for someone to prove me wrong :-)
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Mick Sheppard
Powerbook G3 (firewire)