BelArgUSA
...Passenger seats - as approved by the airline operator, are the only seats that can be occupied by non-crewmembers. Passengers are not to occupy a F/A jump seat...
That may well be a restriction in your company, imposed either by your company or your regulatory authority, but there is no such general restriction under JAR/EU OPS.
In my company, operating under JAR/EU OPS, the carriage of a passenger, on a cabin crew jump seat, is explicitly permitted, at the sole discretion of the Captain.
Such use is very rare, but it is done.
...i.e. to increase the "passenger capacity" of the cabin....
Certainly one may not do this if it would result in an increase in the number of passengers beyond the maximum certified passenger number for that aircraft type.
However, increasing beyond the passenger capacity of a particular aircraft configuration - where (as on our aircraft) that aircraft is configured for fewer than the maximum certified number of passengers - is not a problem, provided that each passenger can be seated in a seat approved for take-off and landing.
Best Regards
Bellerophon