Does the Wagner Flexio 570 have an attachment to use a 5 gallon bucket for the paint supply?

Don't think so...I doubt it would have the suction for that. I would paint a porch with one but not a whole house.

No there is not. They do sell a spare spray unit for about $35.00. A spare unit would give you about 3/4 of a gallon avail. Empty one unit and attach the second unit in mere seconds. Then repeat when they are both empty.

No, because it works under pressure not suction

Unfortunately due to the way this type of unit operates there is not an accessory that will allow you to draw paint out of anything other than the provided container. This unit requires a tightly sealed and pressurized container to establish the flow of material which cannot be achieved through a suction extension.