Hmm... nope. Cut, glue, done. At least, it's physically simple to do. It requires some thinking beforehand though, if you want it to work well.
It really depends on the gun and the pumps that are being used. On a gun like the Flash Flood, you could replace the stock pump with a longer pump without too much trouble. If you wanted to put in a pump with a larger diameter, this would require some cutting.
Also, you have to realise the increasing the pump stroke length by 4 inches increases the gun length by 8 inches when the pump is fully extended. Most stock guns have a pretty sensible length so that people can pump it easily. That being said, there are many exceptions where the physical design of the gun limits the length of the pump.
Now this is where things get tricky. When you replace a smaller diameter pump with a wider diameter pump, you get more volume/inch of length. So if you pull at the same rate you get more water, right?
Well, no. This is where the gun's internals matter. If the tubing for the intake and all the parts (check valves, joints, piping) is too small, and restricts the physical internal size that water can move through, you get some flow resistance, and this is where you can get some gnarly pump lag. Additionally, this increases the force needed to push the increased volume of water (regardless of the internals flow restriction). Also, in an air pressure gun, having a wider diameter pump would make it more difficult to pump to the same pressure as before. It would take more force to achieve the same pressure as a stock gun.