Stop asking, they can't do this.
The Neatworks database is a very simple spreadsheet like contraption. Multiple categories/subcategories, folders/subfolders, etc just aren't going to happen with the internal structure as it's currently defined.
In addition, you would pull your hair out if they did implement it. Why? Glad you asked. Neatworks stores all of the previews of each receipt inside that same database as all of the data instead of as separate jpeg files (Paperless) or a separate database (Lightroom). This is why it takes forever to load your collection as soon as you have a few hundred records. Neatworks has to load hundreds of megabytes of image data, just to start up.
I gave up and created a separate library for each year. It's still glacially slow.
The first thing they need to fix is (as much as I hate to say this) break any compatibility / interoperability with the PC version. Then design the database in a modern relational way instead of as a flat spreadsheet. Dump the images out of the database. Then Neatworks can grow and do all the things we want it to do :)