I have a hunch that the previous posters in this thread must have misunderstood your question.
So, no Marketplace Add-Ons are Transferrable!
Meaning, if you bought an Airplane through Marketplace and you had the Gamepass Version of MSFS, you CAN NOT use that same Airplane on the STEAM Version of MSFS should you decide to switch from the Gamepass version of the Sim to STEAM Version…