You leave the presta valve cap open on the tube. Then you screw on the valve extender. This is the way it stays all the time. You should be able to pump up the tires like that. It works.

The alternative to this (once you've bought them) is to buy some super long valve tubes made for these types of tires, like these:

http://www.biketiresdirect.com/produ...7831&c=1529551

80mm valve, which is about 3 1/3 inches long!