Extra batteries in a Melex??

[Bulky]

My cousin has an old electric Melex cart that is all stock. Runs mint. everything works fine. We also have 6 good used extra batteries. Is it possible to add an extra couple of extra batteries to the Melex? What would we gain from adding an extra battery or two? Not realling looking for more speed, more interested in longer run time....but extra speed would be nice too. Does it hurt by adding batteries?

Thanks for your help