We do not recommend extending this hose for the following reason. 

Although there is a water tank in most portable air conditioners, there is always moisture left in the warm air that needs to be discharged.

If you lengthen the hose, this moisture in the extended hose may have time to cool and condense with the risk of the water flowing back into the air conditioner. This could possibly be detrimental to your air conditioner in the long run.