Well, there were some discussions in blogs regarding Push-to-Talk feature in Skype. Though it is possible to configure a key shortcut to mute-unmute in Skype, it is not actually a PTT as such.
The solution is again to use MorphVOX Jr. This time however it is plain and straight forward, you just disable Morph and Listen in it's main window (click on small green buttons) and set 'Screaming Bee Audio Driver' in Skype as Sound Source (see pics in previous post).
Of course you should configure Push-to-Talk button in MorphVOX Jr options (I use Left Control).