Katy is right, technically it is for non Catholics wishing to find out more. I think each Parish does it a bit differently so it is probably worth chatting to the Parish Priest but I think, in practice, lots of Catholics who wish to understand more can benefit. I attended one which was a real mixture of people and everybody benefited from everybody else's experiences.
Yes! I am pretty sure they would be happy to have you. If by some odd fluke a weirdo said "Oh, but this isn't for cradle catholics..." then I am confident the next parish on down the road would welcome you. I have gone to RCIA as a confirmed Catholic and I've seen others doing the same thing.
Technically, RCIA is for unbaptised catchumens, but they use it for those converting from other Christian denominations who have valid baptisms. It's just the general catechism class.
Nobody is required to join the Church, either. People who are just curious are perfectly welcome.