@Ricardo44:
If a reservation is extended during the time of the original reservation, guests don't have to pay an additional cleaning fee.
There is however no way a guest can avoid paying the additional Airbnb Service Fee for the nights added. Does that make sense?
If the reservation has technically closed by the time the guest wants to extend- you could send the guests a special offer which extends their stay by X amount of days times your nightly rate and you could elect to waive the cleaning fee.
For example, if your nightly rate is $100 and your cleaning fee is $50 and guests wants to add 3 nights, you would send them a special offer for $300.