That's great! Since you have a commercial rotorcraft and private single engine land, you should be able to get a commercial multi-engine instrument add-on rating very quickly! For the Airline Transport Pilot (ATP) multi-engine land you need 1500 hours total flight time with a good portion of your helicopter time that will count! (check the exact requirement of PIC, X-country, instrument, etc.) I think the regional airlines will probably relax their fixed wing total time if you get an ATP multi-engine land. Best of Luck!