You can add catch-all alias manually with MySQL command line or phpMyAdmin tools, for example:

# mysql -uroot -p
mysql> USE vmail;
mysql> INSERT INTO alias (address, goto, domain) VALUES ('your_domain.com', 'user1@example.com,user2@test.com', 'your_domain.com');

If you have iRedAdmin-Pro-MySQL (a.k.a. iRedAdmin-Pro for MySQL backend) installed, you can manage catch-all alias in domain profile page, under tab “Catch-all”. Screenshot for your reference: