Download the Darkwinter Version of the game from the official website.

https://gf2-us-cdn-launcher.sunborngame.com/prod/download/launcher/1.0.0/GF2_Launcher_pc1_1_0_0_OverSeas_Mica_1732302259_6_1000005.exe

or

https://gf2exilium.sunborngame.com/main


Install the game, assuming you don't have it installed already and have been playing it for months with your account (which was my case)

Install the game from the Steam Store, once you have it all downloaded, navigate to your standalone install folder

Normally the default directory is:
C:\GF2Exilium

Copy all the files inside the GF2 Game file:
C:\GF2Exilium\GF2 Game\GF2 Game

Paste them inside the Steam installation folder, default usually is:
C:\Program Files (x86)\Steam\steamapps\common\EXILIUM


Run the game through the Steam library, it should connect to the Darkwinter servers and even have your account login info saved if you were playing it standalone already

Now, you're able to track your playtime through Steam, use the Overlay and take screenshots from the Darkwinter version.

You'll just lack the launcher, but the game itself updates on the start screen at login so you also won't miss any patches.

Be alert, Steam might replace your files in any update made through the Steam Library Downloads or if you Verify Game Integrity!


Have fun!