there are three ways of getting long valves;
a) use tubes with long valves
b) use tubes with removable valve cores together with an internal-fit extension
c) use tubes without removable cores together with an external-fit extension
IMHO this is in order of preference too, (although long valve stems are usually brass and are often so heavy they can throw a wheel out of balance)internal fit extension for tube with removable coreexternal fit extension; (some have dustcap threads, others don't). These remove using an allen key
The problem with external-fit extensions is that in order for them to end up 6.00mm external diameter (and therefore pass through any
presta rim drilling), the wall thickness can't be any more than 0.4mm above the dustcap threads (which are 5.2mm). Often they end up a bit wider than 6.00mm. In addition the valve core locknut can only be tightened if the extension is removed and refitted every time the tyre is inflated.
The problem with internal fit extensions is that they only fit tubes with removable valve cores; these are made by Continental, Schwalbe, Vittoria and one or two others. This arguably defeats the main object of using an extension, which is to be able to use more widely available tubes. However, armed with as many extensions as spare tubes, you are reasonably assured of getting home without a lot of hassle; the extensions will usually pass through any rim drilling.
BTW it is not without risk to open up the rim drilling; the valve should be
supported by a small ridge at the base of the valve stem (and if it isn't the valve stems can be torn out of the tubes). This ridge is only a little over 6.5mm diameter, so if you open up the rim drilling appreciably then the valve stem probably won't be supported properly any more.