Nope, you dont need any pumps. Gravity does all the work for you. Here is a list of what supplies you will need.
-Bucket (try and find a square one)
-Half inch rubber grommet
-Half inch barbed elbow
-Half inch tubing
-Male and Female Hose connectors with Half inch barbed connectors to connect it to hose.
You could also get the drain kit for a waterfarm from GH. You basically want to make a bucket look just like the larger res bucket of your waterfarm. Just the bucket with a drain hose at the bottom. Then you can use this spare res to mix your solution. So when you are ready to add the new nute just connect the male and female hose fitting the you have put on your waterfarm and you spare res, And LIFT the spare res above the waterfarm and the water will drain in from the bottom because of gravity. The nute will enter the res without moving or damaging them at all. You can get all these parts from any hardware store or you could get them from your local shop. Hardware store will be much cheaper. The absolute best way to go is to find the same size bucket as the waterfarm and get the waterfarm drain kit so that you can get one of the blue tubes they use with the marking on it so that you have the exact measurement of how much water to fill the res to where you want it. This way you never have to separate the medium from the res. Sorry if this all sounds confusing, dont hesitate to ask anything you need to know. Good luck!