Hi Esasias,
Thanks for the clarification and as BossDweeb has indicated, that your Product Key applies only to MSFSX and cannot be transferred or used to activate on Steam. As well, there is no Digital version of FSX. The only product that closely resembles what
you are requesting requires a purchase: i.e. FSX-Steam Edition (which is FSX Deluxe with the Acceleration Pack). At the moment, selling for about USD 25, though it does go on special from time to time. This is only available in download format and requires
no activation (except for the Steam registration) and can be re-downloaded again and again (or, saved on a thumb drive) if your version is corrupted in some way. There will additional purchases for extra aircraft, scenery, etc that are not part of the FSX
Deluxe/Acceleration release.
https://store.steampowered.com/app/314160/Microsoft_Flight_Simulator_X_Steam_Edition/
Of course, if you are wanting to purchase another Flt. Sim, then Prepar3d and X-Plane are extremely good productions.
https://www.prepar3d.com/ (USD 60)
https://www.x-plane.com/ (USD 60)
Or, if you're patient and have a fast computer (both CPU and Graphics card), then Microsoft's New Simulator is due for release later this year. Though limited details are available at this time. https://www.flightsimulator.com/
Anyway, the above info is there for your consideration.
You could try having your DVDs copied onto a USB drive and then try and download it onto your hard drive. I don't think you will have success if you took a copy of FSX after it has been installed, because of Activation and Registry issues. But worth a
try.
Chris