I converted some Hella FF75 lights to 55 watt HIDs a couple of years ago on my Xterra. They worked great.
LONG ANSWER: (scroll to the bottom if you want the short answer)
The basic laws of physics are applied here:
Let's look at a 100 watt incandescent halogen light bulb, typical of what you might find in an automotive lighting application. This light bulb uses 100 watts of energy. 80 watts is being converted to HEAT. The other 20 watts are exiting the housing in the form of LIGHT. (Note that these numbers are approximate). That means 80% of the energy is being wasted in heat.
The heat component is what you are worried about, right?
Well, HID lights are more efficacious (no, it is not 'efficiency', it is 'efficacy', look it up). Let's assume that 60% of the energy put into the HID lights comes out as heat since they are more efficacious. Well that works to our advantage, now we have 33 watts of HEAT, and 22 watts of LIGHT. So even though there are fewer total watts, there is more light coming out of the fixture, and LESS heat.
So theoretically, if you have an offroad light fixture built for 100 watt halogen lights, you could put more than 100 watts of HID lighting in the fixture and not worry about overheating it. Unfortunately, that would put out excessive amounts of light, and no one seems to mass produce such a thing. 55 watt HIDs on the other hand will put out less heat, and are guaranteed to not be a problem.
IN SHORT: Yes, go for it, enjoy the light, and don't worry about the heat!