Take a look at those aluminum pool cleaning brush handles. Ditto for aluminum extendable paint roller handles. Either of those might work for ya. So far as I know, PVC doesn't do extendable, but you can certainly do it in sections with the appropriate fittings and a small set screw or two to hold the sections in place. That is what I did for my poling stick. Here are some pics of my "Law Stick", also known as a poling stick for push-poling on the flats and for pushing off when getting underway. Florida Sportsman Magazine publishes a "Law Stick" sticker that not only has a ruler for fish measurements on it, but also the size limits for all the game fish down here. I put the sticker on a 3/4 inch diameter, 5-foot long PVC push-pole that normally resides in my portside gunwale bungee, so that it is easily accessible.
Closer view of the Florida Sportsman LawStick.
An example of some of the species size limits on the LawStick.
Here is the 4-way fitting at the base. Believe it or not, it works great as an emergency paddle as well! The bottom is cut off at a 45 degree angle, so that it sticks into the bottom sediment for traction. The upper portion is the 5 foot extension that can be added to the lower portion by simply removing one screw, then adding the 4-way fitting to the new end, and making the overall length 10 feet.
Here is the T-handle at the other end.
Also makes a good standing/walking stick at trip's end for steadying oneself after 3-4 hours in a cramped sitting position.