Sure - you can do it a couple of ways.
1. Wigs are made of wefts - bunches of hair sewn along the edge. You can make a tiny weft of the highlight hair you want and sew that in between some existing wefts (just google wig weft making for instructions) or
2. Most wigs are made of the wefts being sewn onto a mesh type of fabric. Don't know if you've ever done a latch-hook rug, but you can add hair by using that same technique to knot your streak around the mesh.
http://www.woolneedlework.com/latch_hook/how_to_latch_hook_instructions.htm This explains how to latch hook. Mind you a latch-hook would be far too big to do this on a wig mesh, but you could probably do this with a needle and thread as well.
There are probably other ways as well