USB and FireWire are very different in how they operate (it's not just a matter of the connector's shape), so there is no such adapter.
There's nothing you can do with that external drive, in terms of playing DVD movies from it. If you are handy with taking things apart (and putting them back together), you can probably replace the internal drive, which sounds like it is a CD or CDRW drive (it can't read DVDs), with a DVD-capable drive.
There are take-apart guides online, such as
http://www.xlr8yourmac.com/systems/imacg4/imacg4takeapart.html
(please read it carefully and make sure it is something you can do before trying it)
Getting to the optical drive is beyond normal user access, so it is not a simple procedure. I have not done it myself, but an optical drive like this will probably work (and it's a
super drive).
http://eshop.macsales.com/item/Pioneer/DVR117DBK/
Or you can get an external drive that uses FireWire
http://eshop.macsales.com/shop/firewire/optical-drives/