You can't "convert" a String to a MovieClip, a String doesn't know anything about MovieClips. Also, Strings are Objects. They are just immutable, like in Java.
Anyway, the important part here is that a String is not an instance of a MovieClip, it isn't a symbol name and it isn't a class name. It is just a String.
You can use Strings to find existing instances in various ways and you can use Strings to look up Classes for use in creating new instances. But they are not the same object.
Signature: I wrote a pair of articles about the timeline