Can I chain i-TTL extension cords?
Yes you can. Those coiled cords are quite stiff and heavy, they do not extend as you would expect: twice their nominal length is as much as I dare pulling on those.
I just had to shoot with long iTTL cords a few days ago: my 2 hacked coiled cords (cut, spliced with multi-conductor cable) were connected in chain for a total reach of 25 ft. No problems.
This 3rd party manufacturer makes longer cords, up to 30 ft.
All photographs are only more or less credible illusions