Ok here we go. First of all I want to say I'm no expert on clothing, so if there's a better way to do this, don't doubt to ask or take anyone else advice too, this is just what works for me and what I like doing.
1- First, you should clone an item that is similar to your TS3 mesh, this not only for importing but also for the weights and UV_1 (Which I don't cover here since there're tutorials in the forum already). In this case, I cloned this OUTFIT. It's better to clone something that has the same behaviour you want. In this case, an Outfit rather than a top or a bottom.
2- Now you should export the mesh as always and open it in blender. You will see a 3 group outfit with different cut numbers, which is one for the outfit, and two for the legs. You will use this for the weight transfer and for the UV_1, but you will notice that you don't have enought body (mesh) to use for your conversion. For this reason, I also exported the naked top and bottom mesh for kids. We will use it later. For now, from the 3 groups we have delete the one that has the dress, you will only see the 2 remains of the legs.
3- Now it's time to import the TS3 mesh. When you do it you will see something like the picture below. As you see, there's not only the mesh of the outfit but also the limbs of the TS3 body. We don't want that remains of the body, so we need to delete them. Go to edit mode and then what I like to do is select face mode and press "L". You need to point your mouse to the face you want to select and press L. After you have everything selected press "X" and delete faces.
4- Now, have to import the top and bottom naked mesh we exported before. Select both and join them by pressing Ctrl+J, go to edit mode, press "A", then press "W" and remove doubles. This will make your base for the outfit. This way you have the TS4 body to use. (I hided the conversion outfit while I worked on this)
5- Now you will have something like this. The TS4 body for kids in the back and the converted outfit above it. As you would see, there's a lot of the top of the body that is already covered by the outfit mesh, this parts we will have to delete since they will make room for the conversion UV.
6-To delete the parts that are covered I will do it selecting the UV map, this is the way I find easier to do. You must activate the sync between the uv map and the 3d view. It's the icon with a cube in the uv map part.
Then, select the parts you want to delete and press "X". This would get rid of most of the things, however, go to the 3d view and ajust here and there to make it perfect. You might have to move some verts also so the outfit doesn't clip into the body.
7- Now we need to change the UV name of the conversion, we will join the mesh of the outfit to the mesh of the TS4 body, and if the UV maps have different names we will lose one of them.
8- Then, go to the UV map viewer, select the uv of the outfit and choose the texture of the sim body to see where to replace the uv map. As you will see, the uv is deformed because textures in TS3 were square images. This have changed in the TS4 and now we need to ajust it.
9- To ajust it exactly, press "S" to scale, then press the "Y" key to lock into the Y axis. Then enter 0.5 on your keyboard. This should make the uv map compress in the Y axis and make it right again, won't look deformed.
10- Now, you have to move the parts of the uv into the correct places. EA has a specific place for each part of the sim CAS items so you will have to find where is the correct place. In this case, we have space from the parts of the body we deleted so I moved the uv there. You can see in the picture how I did it, however, I did it in a rush and not thinking a lot. You can play with it until you get a good place without loosing to much quality by resizing. Just make sure to follow the templates.
HERE
11- Then, I went to Photoshop and modified the texture I exported from TS3. I cut the parts and replace them in how my UV is now. Again, you can play with it until you get it perfect. You will have to do this for every texture color you exported form TS3. Or create your own, that's up to you.
12- Finally, make sure that you have the right cut numbers in blender. The outfit should have 0002 and the 2 of the legs you didn't touch and should have the cut numbers correctly assigned. Import to Studio and import the Diffuse texture. I use .png format.
That's it! The UV is done. You need to make the UV_1, the Weights, Vertex paint and the other textures. You have tutorials in the forum for that in case you need them. I really hope you can make it yourself too and If you have any question feel free to ask. Good luck!
