In Europe, Perwazee, we cannot "jumpseat" in the US manner even on our own airlines. However we can sit in the jumpseat if we have a standby ticket and the flight is full.
eg KLM pilots have to pay Z fares to travel (within europe or further afield) even on KLM flights.
So I doubt you can ride without a ticket on Icelandair if that is what you mean by jumpseating.