I know transmissions, gearboxes and reductors convert high speed low torque input to lower speed higher torque output but are there devices that do the opposite?

If you are referring to a system of gears, then yes, it's just like the opposite set up of a system where high speed low torque is converted to low torque high speed.

So for instance, if I had a 3 hp gas engine that ran at 1800 rpm and I wanted to turn a generator that required a 30 hp engine, I could use this system, (combination of gears) to convert the 3hp 1800 rpm to 30 hp?? Rpm, then use a belt and correct size pulley wheels on both the system with the higher 30 hp output and the generator to step up the required rpms of the generator?
Is there a name, other than gear box, for a system that converts the high speed low torque to low torque high speed?

Yes, some research brought me to overdrives:

http://en.wikipedia.org/wiki/Overdrive_(mechanics)

I hope that this was helpful :)