By default games acquired through the Windows Store, including those on Game Pass for PC, install to unchangeable read-only directories, making them impossible to mod. However, some games have a toggle to alter this, meaning the Game Pass versions of FTL, State of Decay 2, and a handful of others actually can be modded. And so can Skyrim Special Edition, one of 13 Bethesda games recently added to Game Pass, and a game that's improved significantly by mods.
However, it's not all good news. The Game Pass edition of Skyrim Special Edition has a different executable to the Steam release, and is incompatible with the Skyrim Script Extender (SKSE). Without SKSE, many of the most popular and ambitious mods won't work. According to the SKSE webpage, it may stay that way. "SKSE cannot support any potential Windows Store release of Skyrim", it says. "Windows Store applications are locked down similarly to consoles and do not allow the APIs necessary for script extenders to work."
So no SkyUI or Legacy of the Dragonborn then. There are still great mods out there that don't require SKSE, however, like those collected at Bethesda.net. If you've got Skyrim through Game Pass, you'll still be able to use Alternate Start—Live Another Life, A Quality World Map, Inigo, and The Forgotten City. Here's our list of the best Skyrim Special Edition mods to help you find more, or see what you're missing.