I though I would create a How To Guide to make everyone's lives much easier and less painful should something happen. First, a general PSA. DO NOT USE MERGE, unless you really want to lose your secondary account and have everything moved to your primary account. I have disable this option for the moment though. Now to how to use this mod.
1) Click on Profile from Menu and scroll to Manage Sub Accounts. Click on the MSA option.
2) Click on Create | Link Account and you should have an three fields. Enter your Username, and current password of the account to link it to your primary.
Should look like this when your finished.
If you forgot your Username or password, an admin can reset the password quite easy and get your username.
3) Congrats on linking your first account as a sub account.
(Further Note: You can create brand new accounts the same way with a unique username.)