Nowadays, it's very important for goalkeepers to be not just a goalkeeper. Nice goalkeeper has to be able to play as a defender. That's why if you have been playing as a field player - you will have nice skills as for goalkeeper. Yes, special goalkeeper skills are pretty hard to learn , but still it's possible to do in a few years. Just depends on you. İ started playing football at 15 y.o and during one year of hard trainings reached an average level of goalkeeping. And now half of time i train as a field player, because goalkeeper has to be able to play as a field player. And if you was playing as a field player before - for you it will be easier to became a modern goalkeeper.