The problem with the word "convert" is that it carries the connotation of rejecting one religious belief and adopting another. That is not how Scientology works. Most Western and some Eastern religions are exclusive, meaning if you are in the religion then you can't be in another one. So, for example, if you are a Christian you can't also be a Muslim. Scientology is not an exclusive religion, instead it is inclusive, meaning that you can be a Scientologist and member of another religion.
That's a difficult concept to get your head around if the only religions you are familiar with are those originating from the Middle East, such as Christianity, Judaism or Islam. Scientology is a religion of wisdom rather than belief and it contains no dogma about God (or Jehovah or Allah or Godhead or whatever other term you want to use for that concept). Instead it contains knowledge, principles and workable practices for living life and improving yourself spiritually. Because it deals with fundamentals that most religions barely touch upon and it does not make pronouncements about the things that most religions do, then it is possible to be a Scientologist and a member of another religion.
A good description of this is given in the Scientology FAQs: "Scientology seeks to bring one to a new level of spiritual awareness where he can reach his own conclusions concerning the nature of God and what lies in store for him after his present lifetime. Thus, like many Eastern religions, salvation in Scientology is attained through personal spiritual growth and enlightenment." (from: Does Scientology have a concept of God?)
So, next time you read that some celebrity has "converted" to Scientology you will know how much hard research the journalist reporting the event has actually done and you'll be able to evaluate how accurate his reporting is.
