Erm, I think you’ll find that a catch all will catch all unknown email addresses but I’m not 100% sure. I haven’t gone too far in that (yet) as I’m still setting my server up, but that’s how I understand how a catch all works; you specify a single email address and any unknown (unregistered) emails on that domain will all go to the catch all address.
I managed to do this pretty simply. In Virtualmin > Edit Users > Username > Email Settings > Additional email addresses, I added @mydomain.com, without a username before the @, as an additional address. Now, whenever someone sends an email to whatevercrazyaddressnomatterwhat@mydomain.com, it goes to the email account of the user I set this up under.