I can say that at one time I ran expedite freight in a F-350 pickup with a topper. The same truck was used for snow plowing in the wintertime.
Anytime there was a storm coming at home, I would go out of service, pull off the topper, hook up the snow plow, and make (more) money.
I was contracted with CTX at the time, they didn't like it when I went out of service, but that was their problem, not mine. I think they were more worried that I had quit than anyuthing else.... since I was going out of service for a week or two at a time.
I say if your "regular" work is on a regular schedule, you could make a go at expediting. However, to make it you need to be out at least a week at a time. Otherwise, you need to look at local delivery work rather than long-haul expediting.
My normal schedule would be to get a load on Tuesday, and be out through Friday or Saturday. At least at the time, there was rarely any expedite work going out Sundays or Mondays.