Nope. There is no way to "make decisions" based on what you have targetted. Which is too bad, really, I've wanted that ability. (But the devs really want to make "bots" really hard to make)
The closest thing would be a keybind that targets the closest ally, then casts Speed Boost, and another key that targets an enemy and casts Siphon Speed. I haven't been able to make a bind that targets and casts a power work reliably though. I suppose you could do it with the "press down to target, release to cast" trick, but that's a lot of work.
Is it possible to make a bind so that if it is targeting an ally it will cast speed boost, but if I am targeting an enemy it will cast siphon speed ?