yes, on an EV, if you add more batteries you can run the motor longer, you want to keep the voltage the same, but make the amp hours of the pack more, you do this by hooking it up in parallel, you of course want to use batteries that are of a simaler age, construction, and if you can the same brand is nice, this helps to keep the batteries equal as they charge and discharge.
if you have a hybrid car, you can also add more batteries, but often the hybrids computer will need modifications to alow it to make use of the new battery, as it keeps track of the discharge and is designed to prevent over discharge.
Most EV's are pretty flexable when it comes to batteries, it is commen to simply have a large space with strong frames to suport your banks of lead acid batteries, if you want to use a differnt style of battery, simply make sure they will physicaly fit, then weld up another frame to suport this new size of battery, but the best thing in my mind is to upgrade the type of batteries used from lead acid (cheap to buy, durable, short life span) with NiCads, or NiMH batteries of course they are going to dubble or triple the cost of your battery pack, and lithium batteries seem to start at about 5 times that of lead acid, but the more you spend the less space, and the lighter they are, so the longer you can drive on a charge.
I was looking on ebay and found some used EV batteries($375) out of 2006 prius. I thought that if I hooked them up to what I had it would give me twice the time on the electric motor. But, if I read some of your post right I can use regular lead batteries also. Is this right?
You gotta find the guy who did the work on the Prius. He was a genius. The kind of guy who has his own patents and such. By now he may be working with an installer to do conversions, or something like that. It's not like you can just get 288 volts worth of batteries and hook 'em up and it all works fine.
Ryland did a great job of covering the question. Since you've said "twice" twice though let me just add that doubling your battery pack size probably won't quite double your range. If you were just running an electric motor with no load, yes. But the motor you're running has to move those batteries, so as you add more of them the weight goes up and you get a "diminishing returns" situation.
How much you fall short will depend on things like the ratio of battery weight to total loaded vehicle weight, and the shortfall may be so small you don't even notice it, but it should be there.
This is just an off-the cuff thought, but maybe: Find out what a second battery pack weighs. Load that much weight into your vehicle. Drive it and determine the range/time with that load. Double that value and that might represent what you'd get with the second battery pack, presuming the condition of both packs are identical. (Anyone see a fault in that thinking? I didn't ponder it long so I don't have a lot of confidence in it.)
Of course if you go with lighter or higher-energy-density batteries, all bets are off.
Not quite making "twice" aside, I like that you're thinking along these lines. The more of us pondering creative approaches to FE, the better the chances everyone will benefit.