No, Unity will only resolve IDs it has in it's database, there's no "free dial" conversation at this point. You'd have to create call handlers for the numbers you want to allow folks to contact from the auto attendant. There is a "bulk handler create" tool that can do this en mass for you - you can get if off of http://www.CiscoUnityTools.com if you're interested.
In some switch integrations you can configure Unity to transfer to a number that essentially just drops you to dialtone off the PBX and lets you go wild - I'm not sure that's a really good idea, but it's an option to look into.
