In the email accounts I create for my addon domains I don't see a way to access a spam folder the same way I do for my normal domains. Is there a way to do this?

The spam box has to be checked either through an IMAP account in your desktop email client (like Outlook or Eudora), or through the Horde mail client. In Horde, there will be a scroll box in the upper right hand corner listing all of the account inboxes. There should be one called "spam". Open this inbox and it will have the SpamAssassin filtered emails.

To check it in an IMAP email client, for the  username use 'username /spam' without the quotes.