You must create a web account before merging your in-game accounts.
Follow this guide to register a web account: https://www.warfareonline.net/news/12/how-to-register-an-account
You may merge up to 5 game accounts per web account
Login to your web account and proceed to account setting page: how-to-login-to-your-web-account
Go to the bottom left corner of the page and click on Merge Existing Game Accounts menu
Provide the Username and Password of the in-game account that you are adding to your existing web account
Note: You cannot merge a game account that has been merged previously - it will give you an incorrect password if you will try to merge an account that has been merged already.