Don't know how big the closet you're talking about is (you don't want it touching doors or walls or anything like that), but yeah, you can heat a whole room with one. I use one of the radiant-oil heaters and keep my whole reptile room 76-80 ambient temp all the time.
Edit: Just be sure you have some way of managing the temp. to keep it from getting too hot. like that thermostat that was linked to earlier