Hi

I've been using unity for some time and i am trying to understand how the Sims allows users to import a texture/normal map to use on a floor or wall mesh.

For my current setup i have a tile atlas, which is fine when its part of the game, but if user imported ones becomes a bit difficult to figure out the right way to do. In the Sims you can paint each triangle in each tile individually (i can already do this). But the only way i see doing it, is using submeshes but its still tricky to rearrange triangles - eg removing them from one sub mesh, adding to another submesh when the user wishes to repaint it.

Is there a smarter way ? 

The Sims has a lot of amazing tech in their rendering engine. Each version has different technology. The game uses some rather complex functionality to allow players to customize colors, texture patterns, and other visual aspects of objects. 

What happens depends on which version you are using. There are some guides like this that may help you. 

Custom content in Unity is an own topic that you'll have to solve. I don't know about current 2018 version but in my old Unity days, we have had to provide extra content as AssetBundle objects. Textures and Normals should be more easy to import while they are bound to certain material/ shader at runtime so you could decide to replace the shader bindings to something else using your new imported texture and normals