I think an "extension (?an old computer term maybe from the early mac days?)" = library
while, if the feature is good enough for Alex to implement into the actual core engine or library, you can always just make your own (core or a part like a magic system ), via libraries.
there's lots of info on how to make libraries, on the wiki.
but basically, all you do is this:
<library>
(your created game code)
</library>
this then enables you to within another game (in~by the GUI~Editor), to add this library file~coding into other game, ready for you to use. So, you don't really need to have Alex change his core engine, as you can create your own (well cores and coding parts anyways at least, though I suppose you could probably make your own engine too as Alex made the entire quest program work as objects so I think this would enable you to code your own quest engine using the quest engine Alex already has, which is how I think he changes~updates his quest program himself. Though I'm a noob and could have this completely wrong, need to ask an actual programmer here or Alex, lol) with libraries.