Currently I believe there is no feature to suction water out of ponds, rivers, lakes, etc.
There are a few ways:
1, connect a tanker into a engine/brush truck to keep it water supply positive
2, get a tanker to drop the portable pond, from there you can dump water from the tanker's water supply, (the portable pond has 3-4 supply connections and the water tanker has one) and then you can supply 3-4 engines/brush trucks with a single water supply, and can keep refilling it with the different tankers