If you are in a situation where you have to move the power source to your pool light and the light cord will not reach to the new location, you have two options. You could buy a whole new light fixture with a longer cord or you could splice into the existing cord and add to it. If you add a spliced cord, you should not put the splice underground inside the light wire's conduit. That pipe generally collects water and the spice would eventually short out. The splice should be made above ground. This guide shows you how to splice into your exiting light cable above ground.
REVIEW THE PROBLEM - A typical situation might be that you want to build on the back-end of your house and have to move the power supply to a point further away from the pool. As A result the pool light cable is too short to reach to the new location. You need to add length to the cable.
DETERMINE CABLE LOCATION - Dig around the base of the conduit below the power box near the house to see what direction the light conduit is going to the pool. Lay out a hose from that point to the pool edge straight off the light fixture to estimate where cable is buried..
LOCATE A SPLICE POINT - Pick a point along the conduit (hose) where you can place a 2' post that wont interfered with other traffic. The best option may be just outside where you are building an extension. Check to see if the conduit is there. You might have to dig carefully on either side of the hose to find the conduit
CUT CONDUIT - You are going to need to bring up about 3' of light cable to junction box. Cut the conduit on the light side of the conduit. Be careful not to cut into the wire cable inside the conduit. You might use a hack saw here to better control the cut. Cut just down to where the saw gets through the conduit. Make several of these shallow cuts around the outside of the conduit until the conduit breaks off. DO NOT CUT THROUGH THE WIRE CABLE.
RE-ATTACH WIRES TO POWER BOX - Connect the new wire to the new power box.