No, it's not possible.
However, if you have such a requirement, perhaps Autonumbers aren't
appropriate. An Autonumber has one purpose only: to provide an (almost
guaranteed) unique value that can be used as a primary key. Negative numbers
fulfill that purpose just as well as postive ones. No meaning should be
associated with the value of an Autonumber field. In fact, it's rare that
the users should even see the Autonumber value associated with a record.