I may be misunderstanding your situation but I think you need to create at least one new project in Flash Builder in order to use your AS classes. After created your porject, you can either import your AS classes using Import -> General -> File System into your project, or set a class path to your classes by going Project properites -> Build Path then addind a parent folder of your classes to Source path.
Flash Builder QE
Ended up just adding new classes into my project and cut and paste the old class code. Wasn't what I was looking for and certainly doesn't reflect any sort of reuse other than the fact that I technically didn't have to rewrite the code.