Clearly there was some magical method to increase aetheric density and capacity, though the method was not disclosed to us.

That method being the one Emet-selch used to make us tangible at the start of the events of Elpis.

There's also the Deep Dungeon methods involving the silver boxes and the aetherpool weapons and armor. Whereby equipping those arms and having them resonate with whatever aether is in those boxes lets them increase their power. And said power directly increases the player's power. These also work in the open world to create the same weapon but in overworld stat terms.

So I guess, as has been the case with all aetherial gear since it was first described from Sastasha, you can increase your aetherial capacity by wearing things with high aetherial capacity, and these same things can increase your aptitude should they have a mechanism that does.