The best option would be to take the tire off and bend the presta head slightly with pliers so it will stay open. Then put teflon tape around it and mount the extender.

Or, remove the tire and buy the Tufo extenders that move the valve core to the tip of the extender.

Either way, the best course of action is to remove the tire and do it right. You don't want to have to mess with the wheels on race day. That will really screw with you to have a flat race wheel 10 minutes before the start.

And teflon tape is like $.99 at the store and glue is $3.00 a tube. $7.00 to fix it right is not bad at all.