Before version 6 the accounts were part of the mailbox selection. Starting with version 6 you can archive accounts multiple times. Therefore, accounts are now separate from the mailbox selection.
Click on the plus icon to create a new account. You will see the sheet below:
Select a template if you have a Gmail, iCloud, Office 365 or Yahoo account. This will prefill the IMAP server. If you click on Other you will have to enter the IMAP server yourself.
The data needed for IMAP are user name, server name and your password. You only need to change the authentication method if your server doesn't support standard authentication. In most cases the first part of your email address is the user name, but not always.
You can archive directly from Gmail or GSuite if you select Gmail as authentication. If your Gmail address is "firstname.lastname@example.org" then your username is "username". The servername is the usual "imap.gmail.com". The server name "imap.gmail.com is prefilled. You now need to enter your app-specific password. The Moth Software blog has instructions how to do this. Gmail's "less security" doesn't need to be used.
Before you can delete an account you have to remove the account from all plans. If you forgot that you will get an error message telling in which plan the account is used.