Is it possible to have a category for Economics and Spanish under subjects?
Yes, but not anytime soon.
New subjects are no longer being launched on Socratic, at least for the time being.
The team decided to suspend this feature in order to focus on developing the Socratic app, which has been the top priority in the past year or so.
The decision was made because launching a new subject on Socratic requires engineering resources that the team cannot spare at the moment.
So, to actually answer your question, yes, it is possible to have those two subjects, but that is not going to happen anytime soon.