If it's a fixed blade and you don't care about looks, you could always go biker inner tube over the handle to help get traction. Cheap fix and easy to remove.
I used bicycle innertube for many years to increase the grip of knife handles and make them thicker when they were too thin for my hand. But then I switched to Plastidip, which produces a hard, rubbery coating. Like the innertube it doesn't last forever, and it may not be the best choice for knives that are going to get banged around a lot, but I really like it and have never gone back to inner tube. I do a minimum of three dips per handle for the best result.
