Changes can be made to the Game Pass version of Skyrim - with restrictions

By default, games purchased through the Windows Store, including Game Pass for PC, are installed in read-only immutable directories, making them impossible to edit.

However, some games have an option to change this, which means that FTL versions for Game Pass, State of Decay 2 and some others can indeed be modified. Like Skyrim Special Edition, one of 13 Bethesda games recently added to Game Pass, and a game that's been greatly enhanced with mods.

But that's not all the news. The Game Pass version of Skyrim Special Edition has a different executable than the Steam version and is incompatible with Skyrim Script Extender (SKSE).

Without SKSE, many of the most popular and ambitious mods won't work. According to the SKSE webpage, it will remain so. "SKSE is unable to support any potential release of Skyrim for the Windows Store," it says. "Apps Windows Store are blocked in the same way as consoles, and will not allow the necessary APIs for script extenders to work."

So no SkyUI or Legacy of the Dragonborn. However, there are some great mods that don't require SKSE, like the ones on Bethesda.net. If you have Skyrim through Game Pass, you can still use mods from there.