If you have your user list in an Excel sheet, you could export it into a "easy to parse" format, like CSV, or even dot-comma separated lines.
You need some lines of PHP that should do this:
Remove all current user that are member of the group that are allowed to login to the portal: this is mine:
http://pastebin.com/uQ6Ry4h0
You should remove all <user>…..</user> that have a that belongs to this group.
Now,
Rebuild the group, and insert all users.
You will have to encode the password (3 formats).
Any of this can be looked up in the pfSense portal PPHP code, so the bigger part of the code is just 'copying' what already exists.
Note that, when a user is present in the User Database (managed by pfSense) their is no need to deleted it, you could also just 'deactivate' the account and reactivate when the "rent comes in".
You could also add MySQL or MSSQL support to the PHP engine of pfSEnse, and use a separated SQL server that will do the authentication (better check how
to generate the passwords then ...)