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First of all, if the station C is enroute station A and B, you can't take a ticket from A to C and use it in the way described even from a booking window.
However, if the station B lies between stations A and C, and you want to travel from A to C, you can buy a ticket from A to C and then do two reservations of it. The first reservation would be combined with the first ticket and second would need to be done separately.
This facility is not available on IRCTC since the e-tickets are only valid for travel with confirmed reservation. So, if you have a reservation from A to B, your ticket will also be from A to B only.
You will have to it from a booking window / agent if you want to avail the telescopic rates.