I tried this last year and had mixed results. I was able to fill the sac using an extension, but I wasn't able to get it to drain.
To fill the sac you can get water through the long extension hose by driving the boat. This helps to force water through the extension hose. However, draining was a different story. The length of the hose is just too long to get water back to the pump in order to prime it. I found it just as easy to use an external pump to fill and drain.
Though, I do use the rear sac from the opposite side of the boat that we are surfing on. I just bought a couple of quick connect fittings and siliconed some rubber stoppers inside them. This way I can take the sac out of the locker, move it to the side where I need it, and then pop the quick connect plugs on. This gives me two GG sacs on the same side for a very nice wave.