https://sims4studio.com/thread/684/changing-body-category-top?page=1
Introduction
This tutorial will show you how to change a CAS full body item so that it will show up in the top section of the CAS catalog and open the top catalog if it is clicked on in CAS. The same method will work the other way around so if you want to use a top and bottom mesh to create a full body mesh you can do that too. It will also work for changing full body items to bottom items.
What you will need
Sims 4 Studio 2.3 (Flora) series or above
Instructions
Save your .package and check it in the game. It will now show up in the Top category and if the model Sim is wearing it and the player clicks on it the catalog will go to the Top section instead of the Full Body section.