Hi bobbybravo,
I am not from Laminar Research; just a flight simmer from down under.
The short quick answer is no you can't.
The steam version of X-Plane is a different package to the original official version of X-Plane created and developed by Laminar Research. The packages do not mix and match.
Steam is a 3rd party hybrid taking a commercial advantage of the development work carried out by Laminar research. Steam installs differently to the Laminar version. There is no connection between Laminar and steam. If you try to order steam through Laminar they will divert you to another site.
You can update the Laminar version as soon as a new version is available including private testing versions, Beta versions, special release versions for public testing and finally the official release versions whereas steam only updates a few days after an official release has been made by Laminar.
If you want to support the future development of X-Plane by Laminar Research and your future benefit in using X-Plane then purchase a digital version from Laminar Research. Don't waste your money on steam.
I know that when you purchase a digital version from Laminar Research you will be provided with a product key so that you can run X-Plane. I then understand, but not tried by me, that if don't insert your disc 1 in the DVD drive and start X-Plane you will be asked for a product key to unlock the program. You then apply the "long" number and X-Plane should open up.
With a digital version you need to be connected to the internet at all times as the software performs some "silent" background checks to confirm you have a legal digital copy. If you run X-Plane on more than one machine at the same time then your system will shut down, possibly for good. It has been known to shut down if you don't connect to the internet at all times.
I don't support steam. If you do purchase steam take any problems to a steam forum because you will not get support from this forum.
Glenn