The team announced the move in a statement. No terms were revealed.
Hatcher was the third-round draft choice by Dallas in 2006, and has 113 tackles and 6 1/2 sacks in 62 regular-season games. He appeared in all 16 games last season as part of a defensive line rotation, finishing with 29 tackles.
Siavii was a second-round pick by the Kansas City Chiefs in the 2004 draft. He appeared in 16 games last season, his first with Dallas, and has 56 tackles in 42 career games.
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