This is exactly as it says. You can delete or lock this thread if I am in the wrong posting this, but how to I combine a meshless top (like the untucked types of shirts) to something with a mesh, like a skirt?

There's really no such thing as a "meshless" top. If it was truly meshless, it would be invisible. What you're thinking of probably uses the naked mesh for the top... so you can extract that from the game files and combine it with the skirt mesh to make a full body mesh.

In the game files, there are GMDCs for all those meshes. I'm guessing the one you're probably looking for is called afTopNaked. (All the mesh files are found in C:\Program Files\EA Games (or Origin Games, if you're using the Ultimate Collection)\whatever EP the mesh is in... naked ones will be in the base game\TSData\Res\3D (or Sims3D if it's the base game)\Sims03. All meshes for all ages are in Sims03.)