In Dutch oven or 3-quart nonstick saucepan, heat 1 1/3 cups water to boiling. Add green beans. Reduce heat to medium-low; cover and simmer 5 minutes.
Gradually stir milk and contents of herb packet from pasta mix into green beans. Heat to boiling over high heat. Stir in pasta; return to boiling, stirring frequently (if necessary, separate pasta with fork). Reduce heat to medium; cook uncovered 4 to 5 minutes, stirring frequently, until pasta is tender.
Stir in roast beef. Cook, stirring occasionally, until thoroughly heated.