Since I don't know every piece of software I can't say absolutely that there is no other way. What you do need is some way of transforming the audio and then getting it to Skype; either this will happen through an application "outside" of Skype which you will then need to route to the Skype input again, or you could see if there are plug-ins for Skype for this very purpose.
Or you could get another computer next to your main one where you run Garageband with voice distortion, then take the audio out from that to the audio in on your main computer! How's that for efficient 😀