Multiple-fare ticket
 

I would like to fly from Europe (Poland) to the USA with LOT/Lufthansa. There isn't any direct connection to the city I am interested in (Phoenix). I need to change the plane in the USA. I would like to fly C on Europe-US flight and F on the intra-USA flight (US Airways).

Is it possible to book such a combination on one ticket? Is there any website you can price and buy such a ticket? Is there any price diffrence in buying those flights on 2 seperate tickets comparing to putting everything on one ticket?

I don't think you can typically mix fares on a ticket unless they break the fare, but on UA (and I assume on US), many intra-US flights are sold with C (with seating in F) for just this reason - to allow you to book a C fare the whole way.