Alex I am not knocking you in anyway so please dont think this but "This decision has been taken for operational and engineering reasons".
what is the real reason? would it cost to much just not possible the people that make poles just dont want to change tools for you ? 
Ian
The development process of any new product is an interesting one and many ideas get modified or developed along the way. Our original design for the extension section was based around a Super-Lite extension with a modified top end that would simply push fit onto the end of the SL-X giving a modular extension with the ability to take extra Super-Lite sections when needed, nice and simple. This would have been easy to manufacture and a relatively cheap product to bring to the market as an add-on to the SL-X.
However when we came to road testing this idea, a few problems cropped up. The first was to do with the ease of use. To fit the extension section you would need to have the pole already fully extended and then disconnect the hose and push-fit it on. This would have to be done with the pole fully extended. Once you had finished using the pole you would have to reverse the process before being able to use the pole on lower windows as the lowest clamp would be 10ft from the ground.
By comparison the fully telescopic section could be fitted at the start of the job and you could carry on using the pole as normal until you need to use the extra height. We road tested this idea with a cheap fibre glass clamped section and it proved a much easier concept to actually use.
Then came the engineering issue. Originally we had designed the adaption to the Super-Lite section as a straight push-fit onto the bottom 20cm of the SL-X pole. In the long term this would not have worked though. Due to wear, this push fit would have become slightly loose over time and would have allowed the pole to swivel in use which would have removed some of the efficacy of the whole pole. To ensure a push fit that would be a tight enough fit to use repeatedly the joint would either need to be a taper fit as on the Super-Lite sections or it would need a clamp on the top of the extension.
As the base of the SL-X pole is a straight section the taper section idea would just not be feasible. To fit a clamp would also not have worked due to shape and the make up of the Super-Lite sections.
This left us with the solution of manufacturing a matching fully telescopic section to slide on the bottom, which will be fully compatible with the rest of the pole.
For those that absolutely have to be able to use their Super-Lite sections with the SL-X extension section I am looking at the possibility of have a fitting made to fit to the bottom of the extension section that will allow these to be fitted. This would probably be custom made upon request. I will have to make one and test the strength of it though.
I am sorry that this reply has been a little long-winded, but that is the full reasoning behind the decision. You can see why I just used the phrase "This decision has been taken for operational and engineering reasons" instead.

Whilst the original concept was a good one it was also flawed so had to be improved upon.