yes, you can do this several ways. You can create call handlers for each of the extensions you want available to folks via the auto attendant - these can be configured to dial any extension/number you like - but you have to create a handler for each one.
You can also use Internet Subscribers for these (they do not take a licenses) - the idea is the same as the call handlers but they can also appear in the name lookup handler and have voice names and greetings and such, they just have no mailstore.
You can also use the special System Transfer conversation offered in 4.0(4) that lets callers "free dial" any number and if it passes a restriction table you define, then the number is simply dialed and released.
You can check the admin guide for info on all three of these options:
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_administration_guide_book09186a0080228436.html
