The Catch-All Alias is a forwarding address, and you can't directly make it be a POP mailbox. However, you can still direct mail sent to any address @example.com into a POP mailbox. To do so, just change the destination of the Catch-All Alias to forward all mail to the POP mailbox address in question.
For example, if you normally read e-mail using a POP mailbox at address@example.com, change the Catch-All Alias to forward any unknown mail to address@example.com. You can then read the incoming Catch-All mail just like any other mail sent to address@example.com.