By Simon Evans

MIAMI (Reuters) - The NBA champion Miami Heat strengthened their already imposing roster on Wednesday by completing the signings of free agents Ray Allen and Rashard Lewis, hugely boosting their threat from long distance.

Allen is the National Basketball Association's (NBA) all-time leading three-point scorer and a 10-time All-Star who won a championship with Miami's Eastern Conference rivals, the Boston Celtics, in 2008.

The 36-year-old guard turned down a two-year offer from the Celtics, reportedly for $12 million, in order to join three-time NBA most valuable player LeBron James and Dwyane Wade in Miami.

Allen will be expected to radically bolster a team that was just 10th overall in three-point shooting last season while also helping to open driving lanes for both for James and Wade.

Allen, who will turn 37 later this month, will likely back-up Wade at the shooting guard position but could also play alongside him as an alternative to Mario Chalmers.

Forward Lewis, 32, is a 14-year NBA veteran and two-time NBA All Star who played with Allen at the Seattle Supersonics from 2003-07. In their final season together they averaged a combined 48.8 points.

Lewis, also a highly-rated three-point shooter who has a career average of 16.1 points per game, spent last season with the Washington Wizards before being traded to the New Orleans Hornets who bought out his contract, making him a free agent.

After two injury-hampered seasons, Lewis will be hoping to have a shot at a maiden NBA championship and he is also reported to have rejected better financial offers.

Lewis, who has also played for Orlando, is expected to share power forward duties with Udonis Haslem and Shane Battier.

The Heat now have 13 players under contract and have two places remaining on their roster.

(Reporting by Simon Evans; Editing by Frank Pingue)