500 ??
Well, check the help file for command line parameters.

with /ADD you can create accounts automatically.
Indeed it can't add them from a text file, but is the only way for automatic account creation. A .vbs file can parse account data in the text file and send /ADD commands to theBat.

I doubt however that's the best way. TheBat needs to create a separate account for each of your email accounts, this will result in 2500+ physical folders on your hard disk. A hard to manage mess.
If the accounts are for the same domain, set a redirection on the server so that mails for all are redirected to one mailbox, and in theBat create an account for that mailbox - then you can have one theBat account per domain.

If the above isn't possible, think of some mail proxy that will fetch mails from all the accounts and pass them to theBat. Maybe there is a mail proxy that can work with accounts data stored in a text file.