The ports on an iMac G4 are not easily changed. This is one disadvantage to iMacs, lack of upgradability. Because of their position and configuration they are fixed to the motherboard. I would not expect to upgrade any iMac ever in terms of ports. They aren't made for upgradability.
It does depend a great deal on WHICH iMac G4 you're looking at. Some had USB 2 some didn't. And 15" with a 1Ghz processor, and 17" with a 1.25GHz processor or any 20" has USB2 ports. Essentially anything after September 2003 had USB2.
USB 1 is usable, but it's much slower than USB2. However, the iMac G4s all had Firewire ports. If your hard drive supports Firewire that would be a great alternative. Even Firewire hard drive enclosures are only $20-$50.
http://www.apple.com/lae/imac/specs.html