Unless devs gave you a free Steam key, no. You'll have to repurchase it here.

Only if you have a Steam key.

Generally if you didn't buy the game for the steam platform then steam doesn't think you own it.
You can add your game to your library as "a non-Steam Game..." from the "Add a Game" menu but it won't have any idea you're running it through steam using your steam account.

Perhaps the devs can hand you a key for activation on steam, or maybe you can buy such a thing from them, but if you can buy from the steam store thats the best way to get it on steam(totally separate from any other platform this game is on).

Thank you, i will look into the Steam key.