We use Dovecot because it uses own file with logins and password and it is a very powerfull POP3/IMAP server.
All email users (pathes to mailbox file) by default (if you use Sendmail MTA and Dovecot for POP3/IMAP) will be stored in the /etc/mail/virtusertables, /etc/mail/aliases and /etc/dovecot/dovecot.passwd. It's simple and configurable.
Qpopper does not have flexibility to configure and pointing pathes to mailboxes and not developed at this time.
Why do you need this software?
But ISPmanager supports this software (but its use is highly not recommended and depricated). You can install Qpopper manually by himself and enable old email system style in the ISPmanager configuration file by
POP3 syspop3 parametr (see
documentation).
The old mail system (Qpopper syspop3) can cause problems when using multiple mail domains and the same names mailbox, therefore, strongly recommended to use Dovecot. It is installed by default on all operating systems with different MTA.