When I first started using Reaper, I too wanted to use an external editor, the way I edited in Logic. However, I soon discovered that you can do just about anything you want within Reaper. I don't use an external editor. The general technique is to zoom in on the bit that concerns you, use 'S' to split it before and aft and isolate it fom the rest of the track, then mess around with it to your heart's content (right clicking gives you a whole heap of options and tools). The big advantage of working this way is that it is non-destructive, and you can go back to where you once were.