https://sims4studio.com/thread/106/make-buyable-garment-using-studio

This mini-tutorial will show you how to make a non-buyable Sims 4 garment show up in the CAS catalog using Sims 4 Studio.

1. After cloning the non-buyable item you will see the garment on the Sim in the Model viewer. Click the Warehouse tab.

1. When the Warehouse view opens click the CAS Part line.

2. Scroll down on the left until you see the line that says ShowInUI. Put a check in the box on that line.

3. Click the Studio tab to return to the Swatch and Texture page so that you can continue recoloring your item.

If you have any questions please post them in this thread.