How to manually create a mail account in exim+dovecot and change the password for existing mail?

Question

Exim

There is a server with exim(4.84_2) and dovecot(2.0.9) installed. There is no web interface for managing mail, and mysql is off / not running.
I determined that the mail format is maildir. Found its location /home/admin/mail/mydomain.com/
Found a file with encrypted (MD5) passwords: /home/admin/conf/mail/mydomain.com/passwd.
1) How to create a new mail? As I suppose, mail is not registered in Exim itself and Exim looks to see if there is such a user and its password somewhere. He looks at the password, judging by the config, in /etc/exim/domains/mydomain.com, which is a link to /home/admin/conf/mail/mydomain.com/ and there is a passwd password file.
But in the config, I did not find the path to the mail itself. Perhaps there is such a user, he looks in the same password file as dovecot, but how does he determine where to save mail?
In dovecot's config there is a path to the mail mail_location = maildir:%h/mail/%d/%n. %h is /home/admin because in passwd such way is registered for all users.
dovecot looks at passwords and existing users like this:

passdb {
  driver = passwd-file
  args = scheme=MD5-CRYPT username_format=%n /etc/exim/domains/%d/passwd
}

userdb {
  driver = passwd-file
  args = username_format=%n /etc/exim/domains/%d/passwd
}

Maybe you just need to add a user to the passwd file and thus create mail?
Do I need to restart Exim after this?
2) How to change the password of an existing mail? Just come up with a new one, encrypt it in MD5 and change it in the passwd file? What if the messages are encrypted with the old password?