Right-handers Jeurys Familia, Elvin Ramirez, Jenrry Mejia and lefty Justin Hampson joined the club along with outfielders Jordany Valdespin and Fred Lewis.
Familia, who sports a 98-mph fastball, was 9-9 with a 4.73 ERA in 28 starts. A starter at Triple-A Buffalo, the Mets will use him as a reliever for the time being.
Familia, who is the Mets' No. 3 overall prospect, made his Major League debut in Tuesday's 5-1 loss to the Cardinals, entering the game at the start of the eighth inning. He gave up no runs and one hit. Familia struck out pinch-hitter Lance Berkman with a 97-mph fastball to lead off the inning. He gave up a single to Jon Jay, but then got Matt Carpenter to ground into a inning-ending double play. Familia threw nine pitches.
"I felt good warming up in the bullpen," Familia said. "I prepared my mind and tried to focus."
Lewis hit .294 with 13 homers and 45 RBIs in 106 games at Buffalo. He was selected as the team MVP.
Valdespin joined the Mets for the fifth time this season. He has hit .242 with eight homers and 26 RBIs in 75 Major League games.
Ramirez is up for the third time this season. He was 0-1 with a 7.30 ERA in 11 games.
Mejia, 22, was 4-4 with a 3.59 ERA at three different Minor League levels.
Hampson had three scoreless relief appearances in a short stint with the Mets earlier this season. He allowed one run in one inning Tuesday.
In order to make room on the roster, the team transferred catcher Rob Johnson and left-hander Tim Byrdak to the 60-day disabled list.