You must create a Web Account before merging your Game Accounts.
Follow this guide to register a Web Account: How to register a Web Account and Create a Game Account
You may merge up to 5 game accounts per web account
Follow these steps to merge your existing Game Accounts:
1. Login to your WEB ACCOUNT and proceed to the account settings page
2. At the GAME ACCOUNT section select “Add game account” and click on “Merge Existing”
3. Provide the Username and Password of the 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’ error if you try to merge an already merged account