Yes NuPoint mailboxes can have multiple extensions linked to them.
If you're looking through the web interface, you'll need to click on the "Advanced" section of a mailbox, and then put the secondary extension number in one of the "Alternative Extension" slots.
However, I don't believe you'll be able to use secondary extensions with * in them, since NuPoint tends to use * as the "escape" digit to move to the "Message Center" level.