This one is just off the top of my head, but a gateway thats opened by a series of pressure plates might work. You grab two levers, one for the start of the process(which will also be used to finish it) and one to continue it. Have a small square of 1x1 channels with a pump connecting them all going one way. One lever is connected to one of the pumps, the other is connected to the rest of the pumps. In three of the channels have a pressure plate activating on 7/7 water. Make three drawbridges connecting each one to a different one of the pressure plates. Pull the startup lever, once the water is pumped out pull it again and then pull the lever connected to the other pumps. This is way overcomplicated and unnecessary, but why SHOULDN'T it be done? Huh?
As for the pool idea, get a bunch of 7/7 pools that total a number that is divisible by four. Then, divide the total depth by four, and dig out that many spaces(Including the stairs/ramp) close enough that a pump could get water from the pool of water to the empty channels. Wall the water's path in for best results, you WILL lose water otherwise.
Example: You have a small pond, four pools of 7/7 water. Next to it, you dig out a 3x3 swimming pool, and a stairway going down. You build a pump in between them, then you build walls around the pump and a door so a dwarf can use the pump. Use the pump to pump the water into the channel, it should be 4/7 all around.