rainbow six siege season pass

I would think so as long as it's on the same uplay account.

I'd be inclined to believe that if you buy the Season Pass outside Steam, you wouldn't be able to access its content if you launch the game through Steam... but don't quote me on that, as I have zero experience with such things.

Your Steam key for R6 should activate the game on uPlay too, so I don't see why the season pass wouldn't work as it's the same uPlay account you're playing with either way.

I found a comment on reddit that claims to confirm you can do this

on rainbow six siege I own on steam and bought my season pass via uplay and it worked just fine
https://www.reddit.com/r/thedivision/comments/49vr4a/the_division_on_steam_season_pass_on_uplay/d0v9t6i

Yes, it would work, but I wouldn't really recommend getting the Season Pass. R6S is one of the few games, where Season Pass is basically a booster/shortcut package. Sure you get some skins, unlock all operators (or all new operators) right at the start, and you could play as them 1 week before worldwide release, but if you really love the game, you could play and gather the Renown to unlock every single character.