If one of your traders already has an MT4 accont but this MT4 account is not in his Trader's Room interface you can easily add it. Here is how: 

Login to your broker Trader's Room admin panel

- Click on "Traders" => "Details" in the drop-down button for the user in question.

- Click on the plus sign next to Trading accounts 

- In the dialog box that appears, enter the MT4 account number that you wish to add.