Not telling us the make and model of the headunit is the silliest thing here
However, it is very unlikely that the answer to questions 2, 4 will be yes, and 5 is a definite no. If your unit is from one of the bigger manufacturers, I THINK a gps module is available for some, but for that you'll have to look around the manufacturers website, ebay or similar stores for your answer.
Generally though, head units aren't customisable - they're a ready-made solution for people who don't want to build their own, and the manufacturers don't want the headache of uninformed customers screwing around with things they don't understand!