Add Script to Project at Runtime

Do you really mean you want to import a script file into your project? (why?) You can do that using System.IO, but simply copying it to the project directory will have no particular effect. Or do you want to add a script to a gameobject in the scene? You can do that with AddComponent. Or do you want to import a script file, compile it, and then make use of the functions it contains? You can't do that.