I don't believe that there's a way to get literally infinite hull capacity. However, the base hull capacity values are set in ShipHullStatDefs.XML in Galactic Civilizations III\data\Game, and you can certainly increase those values (there is probably a limit on the value you can input, but it's likely somewhere around at least a few hundred thousand). Be warned that ships made up of too many pieces have the potential to slow the game down. The variable you want to edit is MassCap.
You can also try reducing component sizes in ShipComponentDefs.XML in the same location.
