I am happy you are interested in our product. I hope you find it not only cool for debugging purposes, but a lot cooler for many others. :)
Now, straight to your questions. It is possible to open your existing PhoneGap project inside AppBuilder. There is even a detailed blog post on that matter
which I recommend you to follow. I hope it helps.
Please, let me know if there is anything else I can assist you with.
Share what you think about AppBuilder and Telerik Platform with us in our feedback portal, so we can become even better!