Hi, I'm currently trying to add an additional spell list to my storm sorcerer character but I can't see where there's an option to add more spells than the class is currently capped at.  Is there an option that I'm missing??

I managed it by creating a homebrew subclass for sorcerer (i called it variant storm sorcerer) based on the storm sorcerer. there you can add additional spells granted under the "options" tab on the tempestuous magic feature for example. just fiddle around with it a bit

Yep, home brew is the only option even with swapping spells from the additional spell lists of Aberrant Mind and Clockwork Soul sorcerers. Make a home brew version and add it there