You can't get an adaptor that makes a low speed port into a high one. I can't get an adaptor to attach scsi to USb and hope that it will work at full SCSI speed. No, it will work at USB speed.
USB 2 is a backwards compatible connection, so a USB 2 product should be able to plug into a USB 1 port without a problem (I haven't seen a USB 2 connection, I'm basing that on what I've heard), and you'll get USB 1 speeds. There is absolutely no way to add USB 2 to any iMac out there since USB 2 is a hardware protocal and adaptors can't make a slow speed connection fast.