For small areas, I draw in new hairs on a blank layer.
Technique: In the Brush panel, tick Transfer. Set Opacity Jitter to 0%. This will cause the brush opacity to fade to zero on each stroke. This tapers the hairs off so they won't have blunt ends. Looks very natural. Experiment with the Fade value to get the right stroke length (larger value = longer hairs). Also, sample color from different areas of the existing hair so the new hairs aren't all the same color. If you use a pen and tablet, you can stroke in new hairs pretty rapidly. (Also, I use a soft brush, but you can experiment with that to get the effect that looks best to you.)