One option: Shuttle from IAD to West Falls Church (metro station, on orange line), 40-60 mins. Then From West Fall Church all the way to Greenbelt (metro station, last stop of green line). The bus shuttle right outside would take you to BWI (35-40 mins). Let me know if you have any questions...
You might want to check to see if there is any shuttle bus from IAD to BWI. If yes, it would be great.