As previously stated any speed or agility work is going to be of a benefit, particularly movements that match defensive movements.
Any sort of x runs or shuttle patterns can be done with slides or mixing sprints and slides.
You could even incorpoarte defensive footwork into your pre existing drills, add a point to point slide or sprint prior to a ball pick up and another movement.
Walls sits or yoga like positions (chair, warrior series, etc) all build not only leg strength but the stablizers you would need.
Finally you can google alan stein or his webpage strongerteam. He has great workouts with basketball specfic movements.