This one might be a good question for your local dealer.
However, my guess is, it's going to cost an arm and a leg, maybe two... and your first born. In other words, big $$$ - and more than the original price difference and more than you expect to be reasonable. Plus, if Kia is like most manufacturers, there is no "kit" and the parts guys won't know exactly all the required pieces to make it like a stock Navi anyway (and watch them squirm when you ask).
I'm sure it can be done the fact is most anything can be done with the right information and the funds.
Now the question is going to be
1. Who has the right information?
2. Is it going to be cost effective?
And as stated, I bet that will be your limitation. I'm not saying you personally wouldn't have the funds, I would have no idea. What I'm saying is my guess is it will cost two maybe three times what it would have cost to buy a nav equipped Rio to begin with.
I would however be quite curious to hear what you find out.
One cost cutting thought would be to find a nav unit from a salvage Rio but that in itself may be tricky depending on whether Kio uses the same unit in other models or not.
forget it and just buy a portable GPS like a Garmin or Tom-Tom, etc, I agree with the others if it would be a available it would be a fortune, probably cheaper to trade your car in. Portable unit $100- $200 factory installed Nav x 20 or more!!!