wont work that way sorry .. if you buy the base game on steam you need to buy the season pass there as well ....as both steam and u play use different methods of file updates and installing DLC
your best bet is seek a refund from G2A and buy the season pass on steam if you bought the base game on steam thats the only way it'll work
again the u play version and the steam version of the game use a different method of delivering DLC as well as patches and updates
also G2A is not an approved retail vendor of Ubisoft products
https://support.ubi.com/en-US/FAQ/9/...0000000ekN9CAI