Take a unreserved superfast ticket from the counter. Board the train in any of the reserved classes and talk to the TTE. If berths are available, he would charge you difference in fare and issue you a ticket (Excess Fare Ticket) for the class berth is available.
Since lots of passengers travelling from Bangalore and/or any station enroute should deboard at BBS or station prior to BBS, there is high chance that there would be lots of berths vacant ex BBS.