United flies once daily to AMS from IAD or ORD, so from Phoenix you'd have to make a connection in one of those cities. If you are looking to get Star Alliance miles, it would make sense to also look at options on US and LH.

In terms of adding new flights, it's impossible to say for sure, but it's unlikely an airline would add seats to Europe in the off season. Chances are if UA was going to add another AMS flight, they would do it in the summer to take advantage of higher fares and traffic.