Fully Custom Blocks?

I am trying to figure out if there is a way to add your own custom blocks to the blocks list. Rather than just adding a code block with static content, or adding overrides for the squarespace blocks, I would like to create entirely new blocks, and add them to the block list under a "Custom" section.

I am currently setup with local development, and am trying to add a new block to my blocks folder. I have both my block and config files (ie: {custom-block}.block && {custom-block}.block.conf). I am now trying to figure out how to add the block to the blocks list in the image below. I would like to add my custom blocks to the more, or a new section. Really anywhere would be fine. 

Does anyone know if this is possible? Can anyone point me in the right direction?