I have a window a little far(+-5 meters) from the room i wish to add an portable a/c. Is it possible to use an extractor fan to help pull the exhausted air from the portable a/c?
I wouldn't without first consulting the manufacturer. You are messing with the design parameters. With that distance, you will loose all legitimate warrantee coverage. I'm not saying it couldn't be done but you would probable need automatic variable speed control based on discharge air temperature as well as built in purge delays and base airflow settings. Or you could just play with it and try to hold discharge temp of between 110* and 120*. Good luck. One day you are going to forget, not have enough cooling to the condenser, run the head pressure up and damage the compressor.
As I rarely worry about warranty coverage, I believe you can do as you suggest if you consider a few important factors.
You would have to select a fan with airflow greater than or equal to the current exhaust airflow. How you determine that is up to your ability to fabricate something repeatable.
The other factor is the temperature of the exhaust from the unit. It's going to be hot. Ensure that your fan is capable of operating at the expected temperatures.
I've done as you propose. For testing purposes only, I created a cardboard cover for the existing exhaust hose with a flap within, similar to a doggy door. Gravity keeps the door vertical (closed) and air pressure forces it open. The amount of opening is going to be your reference for air flow. It's crude, but it's a start.
Connect a hose to your proposed fan and attach the doggy door. If the door opens a greater distance, you have greater airflow. I was lucky in that the fan I selected provided substantially more airflow than the unassisted exhaust fan in the unit. Had that not been the case, I would have had a useless accessory fan and had to locate and purchase something else.
I also measured with an ordinary IR thermometer various locations within the exhaust duct. I discovered that the hottest temperature was directly at the opening of the unit. There is moderate cooling along the length of the hose, but nothing significant. The heat build up at the fan was not in excess of the rest of the system and fell within the temperature specifications of the fan.
I cannot recall what specific device I purchased, but the housing and impeller were metal, not plastic, while some models with plastic had much lower operating temperature specifications.