I’m not sure what you mean by smoothing an edge exactly, but you can use the Set Smooth and Set Solid buttons (F9 context) to selectively smooth faces.
I have used this feature, but it doesn’t allow a single edge to stay sharp.
The red line represents where I would like to keep a sharp edge
If I smooth all, this edge is shaded as well.
If the angle between the left and right side of the red line is large enough, the auto smooth will take care of the shading break (will give a sharp apperance to this edge).
(The auto smooth does actually work in this case, but doesn’t give good results in all cases. Hence, this is the reason for my question and my desire to control it manually.)