The 31-year-old midfielder is expected to join the team as soon as he receives his work permit from the United States government.
Powell, who played for Jamaica in the 1998 World Cup, made more than 300 appearances in the English League. He most recently played for the First Division Sheffield Wednesday.
He started the 2002-2003 season with Premier League Birmingham.
He spent most of his career in England with Derby County (1995-2002), which he captained and helped to win promotion to the Premier League.
Powell was born in Lambeth, England and holds Jamaican and British citizenship.