How to convert dresses and hair from other games to sims 4?

Hi. You will need to either google how to extract these meshes from those games or check if someone already did this and uploaded them online. If you won't find anything. which is unlikely, you can use software like ninja ripper to quite literally rip it out while the game is running. I have never tried doing this so I cannot help with this, but you can just google how to use this software and the process will be the same for any game probably.
As for how to bring these meshes into The Sims 4, look up literally any tutorial on converting clothing or any cas item from any game. The process is almost identical for all of them.

I need to see blend and package files to know for sure but it looks like you did not edit a uv_0 map the right way

Yes, either uvs in the uv_0 map were already messed up or it somehow happened in the process.
If the mesh originally had these uvs, download it from another place.

Also, you need to transfer the uv_1 map and vertex paint the mesh with 007F00.

Install Blender 2.78.
Just make sure to do an extra step when transferring weights:
1). Select the reference*, Shift-select your tail, switch to Weight paint;
2). Choose Transfer weights and By name;
3). Choose Clean and All groups

Hi I tried to do a new mesh but now I faced a new problem...


It only shows in the game as if I only changed the texture and so without the new mesh :(

Here's the package file: www.mediafire.com/file/n14advkij5t84kh/Shawn_and_Aiden_Hair.package/file

Ok first of all I’m talking with you guys maybe want to do conversions too:
Follow this incredible tutorial
Like mauvemorn said above you have to find the model
Basically you write for example Attack On Titan XPS models and then download the xps files(or obj, or pmx from MMD)
Then follow the tutorial and if you have issues with the UV follow mauvemorn steps above