unreserved tickets are issued only at the counters of boarding stations
For eg: in your case you'll have to get down at B and buy an unreserved ticket to C
btw you can instead do one thing
on the reservation form (hard copy) fill as:
From:...
more... A To: C
Reservation from: A To: B
This will issue you a ticket from A to C with reservation from A to B
Will work ONLY IF YOUR TRAIN DOES NOT PASS THROUGH C