if you purchase a DLC on Steam, you cannot transfer it to the Microsoft Store version and vice-versa.
https://flightsimulator.zendesk.com/hc/en-us/articles/360017706059-Microsoft-Flight-Simulator-FAQ
If you own Microsoft Flight Simulator on both the Microsoft Store and Steam, you will only be able to access the additional content on the platform used to make the purchase, even if you use the same gamertag on both platforms.