Add On Certificates
For people who hold a pilot certificate, the first thing you should know is that you are not a student pilot (student restrictions do not apply). You are just adding a category and class to your existing certificate. If you are going for the same grade of certificate (i.e. private, commercial, etc.) there is no requirement to take a written test. You will however, still have to take the oral and practical test.
To figure what it takes, look at the particular requirements for the rating sought. If the experience requirement does not say "in helicopters", your other flight experience will count towards the rating. Once soloed for the helicopter, you can exercise all of your privileges, except the carriage of passengers. Your instructor may place some restrictions in the solo endorsement however.