How to aggregate multiple RSS feeds to a single one?
If you want a program to construct a web service by your own, SimplePie may be a good choice. It is written in PHP.
If you just want a tool to make work done, I suggest Yahoo Pipes which works really well for me. (Yahoo Pipes was shut down on Sep. 30, 2015)
If you would prefer an online tool, RSS Mix is an online RSS aggregation.