Bring broth and the water in medium-size saucepan to boiling. Remove from heat; stir in couscous. Cover and let stand 5 minutes.
Whisk oregano, sugar, mustard, lemon juice and vinegar in small bowl. Whisk in oil in thin stream until well blended.
Transfer couscous to large bowl. Add tomatoes, cucumber, Feta if using, scallions and roast beef. Add dressing, and toss to combine. Serve at room temperature or chilled.