The trade is the MetroStars' first since Bob Bradley took over as the coach in late October. The trio played under Bradley when he served as the assistant coach for United's first two MLS Cup titles in 1996 and 1997.
Moreno, 28, is the fifth-leading scorer (69 goals, 52 assists, 190 total points) in MLS history.
Pope, 29, anchored the backline of the 2002 U.S. team that advanced to the World Cup quarterfinals.
Williams, 32, returns for his second stint with the MetroStars, having started 21 games them in 2001.
Petke, 26, is the MetroStars' all-time leader in games (134) and minutes played (11,520).