Gspot,
You can do that in Spain on any carrier. All you need is a valid pilot's licence and an airline crewmember ID.
Just show up at the gate or the aircraft and tell them you would like to go as an EXTRA CREW. It's up to the Capt to take you and most -almost all- would, no questions asked, no paperwork. You can ride jumpseat or cabin.
The logic behind it is that the flight is safer with you on board. Lots of guys use this system to commute.
One final word of advice. Just make sure your request reaches the captain as sometimes
gate agents may not be too cooperative or under the pressure to get the flight out on time. Insist upon it or get to the aircraft via the ramp and talk to the captain directly.
Happy travelling.