If you have a Author Central account already and have some books in it under one Author Name and you want to add a second or third one you can.
Go to the website https://author.amazon.com/ and if you been there before you are probably logged in. If not log in.
You will see all your books under one Author Name here.
Now on the top right, click on your Author Name and it has a little arrow pointing down and click log out.
Click "Join" (Not login) with your normal email account, and yes the same one as the one you used for your first Author Name.
As soon as you are in, you can enter the ASIN of a book that you are creating for your second (or third) Author Name, then click the book to claim it. You may have to click books on menu, and add a book to do this last step.
It may be quick or a few hours, once it has been processed, you will now see on the top right corner your Author Name. When you click on it, you will be able to change to your other Author Name.
You can repeat this above for up to 3 accounts. If you want more than three, you will have to click "Help" and "Contact Us" and ask if they can add another for you. Include the Author Name you want added. They will approve a maximum of 4 more up to 7 total Author Pages.