Hey there. You have cross-save set up between Steam and Epic. Yes, you can buy a season pass on this account while playing on Epic, and it will be applied to your Steam character set. The contents of that season pass will be available to you while playing on both Steam and on Epic. Just to be totally clear, this is for a Season Pass for Season of the Wish. If buying main expansion DLC like Lightfall, that is only playable on the platform on which you bought it - Steam if bought on Steam, etc.