I think you mean...if you were originally scheduled with a 2 pilot crew to say fly JFK-LAX= 6 hours and then at the last minute give you a 3rd pilot and then make you fly JFK-LAX-JFK = 12 hours. If that's what you mean then the short answer is yes they can assuming your airline is authorized to utilize these rules. As far as 32/7 goes it would depend on what you had been scheduled for the month.
121.487 outlines the rule, however...basically if you fly 20 hours or more during a calendar month in a 2 pilot crew then you follow the rules for 2 pilot crews regardless of the number of crews assigned to that particular flight.
121.487 (b)
(b) The monthly flight time limitations for a pilot who is scheduled for duty aloft for more than 20 hours in two-pilot crews in any calendar month, or whose assignment in such a crew is interrupted more than once in that calendar month by assignment to a crew consisting of two or more pilots and an additional flight crewmember, are those set forth in §121.481.
121.481 is...
Flight time limitations: One or two pilot crews.
(a) A certificate holder conducting flag operations may schedule a pilot to fly in an airplane that has a crew of one or two pilots for eight hours or less during any 24 consecutive hours without a rest period during these eight hours.
(b) If a certificate holder conducting flag operations schedules a pilot to fly more than eight hours during any 24 consecutive hours, it shall give him an intervening rest period, at or before the end of eight scheduled hours of flight duty. This rest period must be at least twice the number of hours flown since the preceding rest period, but not less than eight hours. The certificate holder shall relieve that pilot of all duty with it during that rest period.
(c) Each pilot who has flown more than eight hours during 24 consecutive hours must be given at least 18 hours of rest before being assigned to any duty with the certificate holder.
(d) No pilot may fly more than 32 hours during any seven consecutive days, and each pilot must be relieved from all duty for at least 24 consecutive hours at least once during any seven consecutive days.
(e) No pilot may fly as a member of a crew more than 100 hours during any one calendar month.
(f) No pilot may fly as a member of a crew more than 1,000 hours during any 12-calendar-month period.
There are no special rules for relief crewmembers - they are required crewmembers so must meet all the same duty/training/rest requirements whether they fly or not.
Hope this helps.