Sure, it's easy to do, and there are a couple of ways of doing it.
The simplest is to just create additional shortnames for the specific user in Workgroup Manager - so just 'joe' might also have additonal shortnames 'webmaster', 'hostmaster' and 'chiefcook'. Any email addressed to any of these addresses would end up in Joe's mailbox.
Another option is via Postfix aliases where you define a list of email aliases along with an indication as to where emails for that alias should be sent, e.g.:
widgets: joe, bob
foo: joe, email@example.com
Here you can see the webmaster and hostmaster mails to go joe and bob, respectively, while 'widgets' goes to both, and 'foo' goes to joe and an email address at some other domain.
Postfix aliases are well documented on the Postfix site and there are numerous examples online.
I've discovered (now that I've upgraded to Mavericks) that the best way to accomplish this is to create a Local Network User with no Home directory ("None - Service Only") for each email address. From there you can set up the appropriate email account in any email client. You'll need to have Open Directory turned on for this to work.