Yes, but you must buy (or already have) a season ticket for train travel as well. You can only buy a PlusBus season ticket for the towns that are named on your rail season ticket as the origin or final destination oif your train journey (not any intermediate town). 

PlusBus season tickets are now available for nearly all PlusBus served towns and cities across Britain. Each PlusBus served town or city in Britain has its own page on this website. PlusBusseason ticket prices are shown (where available) on the town-specific pages of this website.

Season tickets are available for specific periods only: 7 days (consecutive); a month; quarter year and annual.

The validity of your PlusBus season ticket does not have to match your rail season ticket validity, but your PlusBus ticket MUST expire either on the same date as your rail season ticket, or on a date before your rail season ticket expires.