I can't help you explaining how you would go about adding insulation to the cap, but I would think that it is a major job. The cap would have to be removed which would affect how the roof is sealed around the cap plus much more. I've attached a picture of the 5er I picked up last April on the production line. The pic shows the insulation attached before the cap is installed. I'm wondering how much more insulation could be added from what is already there. It appeared to me that the space between the cap and wall was going to be pretty much filled to begin with. It would be a shame to take everything apart just to find out that the insulation originally installed is as good as it could get. Good Luck.
2013 Surveyor SVF 293
2013 Ford SD F250 4x4 SC; 6.2 EFI V8
Reese 18K Elite w/slider - Ada, Michigan