https://sims4studio.com/thread/13309/smooth-weight-uv-1-transfer

Hey! Me again In this tutorial, I will show you how to do a smooth weight transfer with your CAS mesh. This works best with retopologized meshes. If you have any questions about anything, do not hesitate to ask!

What you will need Sims 4 Studio (I'm using Wishes 3.1.1.5) Blender 2.70 & 2.77

After creating your mesh and doing all of the necessary steps, open your file in Blender 2.77 and append your reference mesh. It's important your reference mesh looks similar to your mesh. After you append it, go over to the mesh you're going to transfer the uv_1 to and add a new UV map. Do not rename it.

After you create the new UV map, go over to your reference mesh and add a subdivision surface modifier and make the view 3, or whatever your computer can handle. The higher the number the more so "smooth" it will be. After adding the modifier, go back to your main mesh and add the data transfer modifier. Make your reference mesh the source object and check Face Corner Data, UVs, and make the All Layers uv_1 and By Name the UV you created earlier. Depending on how high you made the subdivision surface modifier the more slowly it will transfer. After all of this, you can apply the data transfer.

Once you apply it you can rename the UV map to uv_1, delete the reference mesh, save the blend file, and close 2.77. Now you can open your file in version 2.70 and append the reference mesh again. Add the subdivision surface modifier to your reference mesh again and use the same information you used in 2.77. After adding the modifier, select your reference mesh and then shift-click and select your main mesh. The order is important. After you have both selected, go to Weight Paint and transfer weights to the nearest vertex. For more information about weight transfers, please look at this thread.

And here is the before and after! If you'd like to grab the shirt and use it in your game, you can find it here.