Of course, you can do whatever you want. For the flights that I've had that diverted back to the departure airport, I've just put in for example DFW-DFW, with zero miles but with an estimate of the time. Way points is an interesting idea, but if somebody wants to go in that direction, then most every flight could turn into dozens of segments. A compromise might be to figure out where you were when you turned around, and log that in as to/from.