Can you pay for an extra legroom seat when you book with miles?
 

I'm at a crossroads here where I'm about to be travelling more and need to decide if I'm doing it with United or Delta. I prefer Delta, but leaving out of the DC area the miles have limited international utility without going to partner airlines. If I book using Delta miles on Alitalia (and if anybody happens to know, the rest of SkyTeam esp Air France and KLM), can I book a standard economy ticket with miles and then pay with cash to get an extra legroom seat?

Yes, that is exactly what we did. Booked with Delta Skymiles on Alitalia and then paid $110 USD for extra leg room seat.