If you really need 11 speed, your best bet is to buy a new 6800 rear wheel, or wheelset, and sell the existing wheels on CL or EPay. New rears from UK suppliers are $220, wheelsets about $300.
Anything else to adapt shimano 10 speed hubs to 11 speed is a kludge, and usually an expensive one.
You can use the new 10 speed Tiagra 4700 shifters with the 6700 wheels though. These are similar to 5800/6800 shifters, but 10 speed instead of 11. You will need new derailleurs to go with those shifters, old stuff is not compatible.
There is the Shimano Tiagra FH-RS400 11speed hub, it has retained the 10mm through axle.
You could try to screw it's freehub body to your WH-6700 wheel set.
If that works you have to replace the spacer on the left side with a thinner one and center the wheel.
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