The answer
Yes. You can register a .at domain at atdomain.at without a hosting package. You do not need a finished website when buying. Registration does not create a website or email mailbox.
You can connect existing website and email services using DNS records or use custom nameservers. atdomain.at currently sells neither web hosting nor email mailboxes.
What to do
Register the domain and wait for confirmation.
Have the DNS instructions from your website or email service ready.
Select the domain in your account and add the required records.