Aliases and their use are focused in the manual.
Aliases can be defined in a simple dialog which can be opened by clicking on Aliases:
in section
Enter an alias without @
and domain.
Optional item describing the alias. This entry is helpful especially for better reference in case you use many aliases.
The address for receiving emails sent to the alias. Select the place where the messages will be stored:
Email Address — any (user or group) email address. Click to select a user or a group (available only if the group has an email address defined) from the list.
Public Folder — only existing public folders can be selected. If you need a new one, contact a user with rights for creation of new public folders. Usually, this user is the Kerio MailServer administrator.