Is this an external or insump skimmer?
If it is an insump skimmer, you cannot raise the return pipe of the skimmer as it will negatively affect the performance of the skimmer by raising the inside water level of the skimmer. You would need to raise the skimmer instead BUT you must keep in mind that the skimmer needs to be within a range of 6"-6.5" of water,
If you are referring to an external skimmer, the same concept applies. You will need to raise the entire skimmer instead of just making the return pipe taller. If you make it taller, you will end up overflowing the skimmer.
Regards,