If it is for emergency use, consider a jug of drinking water instead. It won't matter if your existing coolant is diluted a bit since the weather won't be below freezing and the concentration of minerals in a gallon of filtered drinking water isn't enough to harm anything, especially not short term. You can also drink the water if you get stranded, or bathe in it, but you can't do either with coolant.
Not all coolants are compatible with each other. Mixing incompatible coolants together will form a sludge so it may be time to flush out whatever is in there so you know what you have. If you have it flushed Zerex G-05 coolant concentrate mixed 50-50 with distilled water would be a good choice.
Yes fill the overflow bottle to the full line. But until you know what's in there I would add nothing but distilled water which is available inexpensively by the gallon at your local grocery store.
It's less expensive to buy concentrated coolant you add your own distilled water to. Make sure to never add plain tap water which will cause mineral (hard water) deposits to form inside the radiator.