Does anyone know of a script that can add accounts to ProFTPd?

Take a look at this

http://www.castaglia.org/proftpd/contrib/ftpasswd.html

But how could I modify the proftpd conf file?

Um, I don't know much about proftpd, etc. But I think it depends on how you have proftpd setup; what type of authentication you are using. You can have it setup to use virtual users - not sure what type of database that uses - probably a DB3, etc. Or of course you can have it set up to authenticate system users, using PAM, etc.