I have no idea why you want to use the same email address for multiple accounts but you can however use Gmail's delimiter (+ plus sign) for aliases to create additional email addresses that point to the same mailbox.
davb+1@gmail.com,
davb+2@gmail.com,
davb+3@gmail.com...
davb+100@gmail.com all point to
davb@gmail.com.
However, there is a common design flaw where quite a few websites don't allow the use of the + character to be used in an email address field, this is wrong and the + character is a valid character as set in the standard
RFC 822.
I'm not sure if XenForo allows the use of the plus sign delimiter, I can test this for you later but I'm sure somebody else will be able to clarify this in the mean time.