The place to start would be to figure out how big a turbine the existing towers could support. If you look at the towers for big turbines, they are big tubes of steel, I guess the cylindrical shape is important (strong). If you look at really big power poles, they are lattices (not as strong).

There are power poles that are sort of in between, roundish with flat sides, but they are usually more medium sized.

So you quickly get to asking why do utility companies put their money towards building large (which helps with efficiency) turbines on the best wind sites, rather than smaller turbines on random sites that they own.

The blades on turbines are massive and would require adding a lot of height to the existing towers. Also working around live transmission lines is extremely dangerous much less trying to add to the tower. It's just too dangerous.

Plus there's the established hazard to wildlife getting chopped in half. A couple months back I read about some kind of passive system involving ionizing water (memory fails; I may be wrong about the method) instead of using turbines. I wonder if building that kind of structure onto a power tower would work.

Ah, found it finally: http://www.ewi.tudelft.nl/en/current/ewicon/

see also: https://www.youtube.com/watch?v=CqEccgR0q-o or search for 'wind turbine brake failure' on youtube. you don't want this stuff anywhere near high voltage lines.