Вот так: http://askubuntu.com/questions/702060/how-to-give-...to-www-data-to-log-in-with-ssh

1. Set a password for www-data:
sudo passwd www-data

2. Define a shell for www-data:
sudo usermod www-data -s /bin/bash

3. Give permission for login via ssh, edit /etc/ssh/sshd_config and add the following line (do not forget to enumerate all the users that need ssh access):
AllowUsers www-data

4. Кроме того, можно создать домашнюю директорию для www-data с помощью mkhomedir_helper:
mkhomedir_helper www-data