Installing ktorrent 3.x will be much more painful than that, since it requires kde 3.x, which is not in the tree either, which in turn requires qt 3.x, which is not in the tree either. If you truly want to keep ktorrent 3.x you could try the kde-sunset overlay, via layman. But I have no idea if it's still alive and I have no idea either if ktorrent is amongst the packages it can provide.
If you have layman already setup (I'll leave that as an exercise, and google can help) then all you need to install the overlay is
layman -a kde-sunset
Then install whatever packages you want from the overlay.