Since DirectAdmin version 1.55.0 allows to manage SSH keys at admin, reseller, user level. Here are some insights.
It's possible to create, edit, delete SSH keys in Directadmin panel now.
Connect to Directadmin as an User -> Find "SSH Keys" and click it:
Here on the image you can see a form for creating a SSH key. Note all the fields are required, the password fields are optional. If you click "Authorize" the created key will be added into a list of authorized keys, so that you can use for connecting to a SSH on the server.
As soon as you create a new SSH key you can copy it to your device over FTP or Directadmin manager under /home/user/.ssh/id_rsa. Where user is your real username. And id_rsa might differ, depending on your Key ID, specified in the form.
Authorized Keys lists keys from /home/user/.ssh/authorized_keys
Public Keys lists keys existing /home/user/.ssh/
You can add additional options per a key, for example it's possible to specify IPs from which an user can connect to server with the authorized keys.
If you click "Global Key" you can specify a list of user for which the key will be installed.