On a three inch extension it would leave the join half way in the middle of the two trees,

with the wall thickness of the fork tubes so thin at the point of the joint it looked a little worrying,

I felt that the canter leaver action at the wheel end would be magnified greatly at the bottom tree causing a prising action and stress to the join between the trees and cause splitting to the walls of the tubes,

What is the thread size and type of the fork tube and fork cap please

Has any one done this extension conversion to a Rocker C please

 

Although I have not seen the extension tube that you are considering, one would expect that it fits snugly with the OEM tube's ID, some significant way down into the original tube, perhaps even down past the level of the lower triple. There is lots of room for such an extension if you look at the length of the OEM spacer used down in there. This would alleviate your concern for a "prising action".

 

Appears you got your answer. But one thought. Be sure you use a 6 point nice socket and put couple layers of black exectrical tape on the nut. Its easy to leave marks if it's tight. Oil will not come out but it's best to hang wheel which will take load off most. Like man said however last few turns hold it down. When hanging mine is not preloaded.

Sorry, should have looked at your picture. Yous is totaly different than mine.