First install the game from steam which is around 600MB more.

• Now open your GF2 launcher game folder (by default the file location is free to be determined by the player but certainly the folder name is "GF2Exilium") and inside that folder there will be a “GF2 Game” folder.
  
• After that Cut the “GF2_Exilium_Data” Folder and move it to the GF2 folder in “\steamapps\common\GIRLS' FRONTLINE 2 EXILIUM” or just right click on GF2 on steam and select the folder location. Overwrite all files in the steam version of the game.
  
• Last verif the game file from steam (if you don't do this the game will crash immediately).

After all that it's just unistall the Launcher version of the game and play game on steam using your existing account. I used this method in the Sunborn/Darkwinter version, the HaoPlay version should be the same.

If there is a question whether the steam version and the launcher version are the same and whether you can use the same account. The answer is yes, the steam version and the launcher version use the same version, only distinguishing how to open and update the game. One opens the game via steam and the other opens via launcher.

In the guide it is explained that after migrating from the launcher to steam, you must delete the usual game launcher, because if you only move without uninstalling the launcher, every time you close or the game asks to restart after the update, the default launcher will suddenly open automatically and it is quite annoying because it will appear windows administrator verification.

For accounts if there was previously a built-in launcher, when logging in it will automatically log in with the same account that was logged in in the launcher version, because it is not related to steam. This GF2 steam version is almost similar to some games that have a launcher and a steam version, for example Reverse1999, which if you reinstall the game after previously deleting it will ask for a login again and not log in automatically. So GF2 can use more than 1 account on 1 steam because the account is not directly linked to steam.