There are plenty of pull-focus devices for DSLR cameras but I don't see anything for zooming in and out with a Canon zoom lens.
I have a Canon 28-135mm Ultrasonic zoom lens for my 5D Mark II. Trying to manually zoom in or out by turning the ring isn't working out well at all - the zoom isn't very smooth. I could, of course, have the lens serviced and lubricated, but I'm thinking some kind of additional hardware is necessary to zoom smoothly while capturing video.
Use a follow focus system. Or, don't zoom, it's an outdated look in my personal opinion, unless of course you need to zoom for a reason that is crucial to the video and there's no way around it. A cheap option for a one time fix is using a thick rubber band that will wrap around your zoom ring and the lens, giving you some extra friction that will make the zoom smooth(ish) with some practice.