Hi Laura
Unfortunately, you can’t, that is if you want to use them as part of the Wix Stores app, however, you can create a separate database and store the main categories and their sub-categories. Take a look at this website developed by me, that’s how I managed to build this mega menu and its sub categories.
Please also note that this is not a Corvid related issue, Corvid forum is a place to discuss issues related to code, other issues should be submitted to Wix Support .