I'm using CPanel with Exim. Is there a way to manually add email addresses to Exim?
Yes, this is possible. You can use /scripts/addpop to add email accounts from the command line. The addpop script will take the following variables:
/scripts/addpop firstname.lastname@example.org password quota
For example, say you have a domain example.com and want to create an email account "admin" with a password "h3LL0th3r3" and a quota of 1000 MB, you would run the following:
/scripts/addpop email@example.com h3LL0th3r3 1000
This script will create a working email account that will show up in cPanel > Email Accounts area.