Just like any other knives, rigging knives can also be sharpened. The procedure depends on the type of blade though. For example, straight blades are very easy to sharpen. On the other hand, serrated blades require more work, but sharpening is still doable.
As for the sharpening method, most experts rely on professional sharpeners, sharpening hones, or sharpening stones. Choosing the right tool is – again – a matter of personal preferences.