Separate ground wire legal?

My home was built in 1964. It appears that all of the outlets (except some that were for an addition) are 2 wire with no ground. There are GFCI's in both bathrooms, however they are not marked as not grounded (required to be marked I believe), and were probably added to pass inspection at one time. I want to run a seperate wire from each ungrounded outlet to the main breaker box (service panel?). Would this be legal? From what I understand it is, but I am not 100% certain. It must hold up to NEC code obviously for when I sell the home in 10 years or so.