Is there a way to add an unread folder to each individual account? I have several email accounts and would like to see the unread emails in each separately.

Hi Nico,
there is no default function for this.
You can set up a Search folder that would take only Unread mails from certain account, but you cannot move this search folder you your account tree, it would be up in the Smart folders section.
Or you could just set up rules to move all new messages for a certain mail account to a specific folder, but then you’d have to move them out manually after Marking them as read.