Dvorak played his first four and a half seasons with Florida, then spent time with the New York Rangers, Edmonton Oilers and St. Louis Blues before rejoining the Panthers for the 2007-08 season.
The 33-year-old Czech native was drafted by Florida in the first round (10th overall) of the 1995 NHL Entry Draft and has 215 goals and 340 assists over 1,105 career games.
Dvorak is in the final year of a contract that carries a cap hit of US$1.7 million.
Bergfors, 23, is in his fourth NHL season. Drafted by the New Jersey Devils in the first round (23rd overall) of the 2005 NHL Entry Draft, Bergfors was part of the trade that saw Ilya Kovalchuk go to the Devils in 2010.
This season, the native of Sodertalje, Sweden, has 11 goals and 18 assists in 52 games.
Rissmiller, a 32-year-old journeyman, has 183 NHL games under his belt, spending time with the San Jose Sharks and New York Rangers before being traded to the Thrashers, along with Donald Brashear, for Todd White in 2002.
The native of Belmont, MA, has only played one game for the Thrashers this season while playing 63 games with AHL's Grand Rapids where he has 20 goals and 25 assists.