Yes Roger, I knew You can be very cruel :) Bad news.
Ken, I am mulling over to design a pole extender which is threaded into pole's handles, or making tube insert for the pole extender which is threaded to the pole's tip, or both versions.
Definitely the best solution (even without having done FEA) would be attaching pole extender at the handle via adapter glued into them and use poles in the shelter in 'tip down' position - much better strength and stiffness due to better load distribution and adapter goes into thicker tubing plus thicker tube of extender. The downside is you need to cut handle tips to glue-in threaded adapter (with new attachments for pole's webbing) and slight increasing collapsed pole's length...
For the version attached to the pole's tip it is again an insert for the extender. The point of this version is to is keep it attached when inserting pole under shelter's canopy or pole sleeves sewn to the shelter, and eliminate extender's play due to pole tips differences, while allowing some inclination variation 2-3deg each side to avoid tip breakage.
Again, Ver.1 is waaaaay stronger and stiffer solution. This is important to me because would be used in the A-frame shelter able to withstand gale winds (I hope).
David, tripod thread sounds rational but needs to be tested for attaching pole extender into poles handle, it can have some lateral play for his application.
If thread at the pole tip is as Roger said this is not an option for Ver.1. Nevertheless I can overcome this... by eliminating thread altogether. As thread is conical and made of plastic with some elasticity I can make 3-4 conical rings inside of adapter where it come into contact with a thread, to click-lock it slightly. But such adapter has to be molded plastic than, or to be made in aluminum it needs to be CNC-milled in two halfs and than glued inside of the extender's tube.
BTW anyone knows what is the thread on top of the GG LT4's where they attach pole extender? Never seen any photos in action.