how to add a game that you already have install and it is not from any platforms. I installed DCS world form original website and now i cant add it.

You're talking about Galaxy, I presume? Just click on "Add games & friends" on the top of galaxy, then "Add game manually", type the title of the game, select it, then click on "Mark as owned". Now all that's left is to click "Link executable" and select the appropriate file from the game folder.