Yes, that is possible. You can create login credentials both on domain as on user level, allowing your customer to login to the SpamExperts control panel directly.
To login as domain user, create a domain and password by clicking the username on the right on top of the SpamExperts control panel.
To create login credentials for a mailbox user, go in the SpamExperts control panel to the button Manage e-mail users. The users will then see a limited SpamExperts control panel, in which they can view their own logs and quarantine system, and change some settings.
To log on directly to SpamExperts using these credentials your customer can navigate to https://spamexperts.openprovider.eu.
Alternatively, you can create a CNAME (or an A) record for one of your own hosts. The server is configured to show the logon page for SpamExperts regardless of the hostname that is being visited!