How to add textures for one mesh?

Is this a CAS or an object item?

Its an object item (sculpt). 

OrangeMittens in your tutorial you convert 2 pillows each having the same texture...But What if the mesh we want has different textures? For example I'm trying to convert the Abandoned Train Station (Midnight Hollow) which has 4 groups (I think) And every one of them has a diferent texture.Is there a way to add more than 1 texture in Sims 4 studio for each group such as the aquariums have? I have arranged everything perfectly in milkshape then I import the .obj in blender and the textures are fine...but when I try to import the mesh it always changes the texture to the one I have in Sims 4 Studio to ALL GROUPS which is impossible to get rid of! The aquariums have 2 textures 1 for the water and another onefor the outside depending on the groups the mesh has....is there a way to do this myself??
Please someone respond
~thanks~

When you are Talking about several Groups of Meshes with Several Textures - That's a 3 Step deal. 

°First, you need to make a Blank Alpha Map that is Large enough to Layer All of the different Textures onto, (This could require some Scaling' to make them all fit.).

°Second, you will need to Map EACH different Mesh part to the new Custom Map that has All of your Textures on it in Blender - Do not Join them, take each different Mesh Group and Map it to your' Custom Map. Do not Join them together yet.  Save each part as a Seperate .OBJ file.

°3rd Part:  When you open the Object in S4S that you are Swapping it out with, IMPORT YOUR CUSTOM MAP TEXTURE into that Object in S4S and Click Save. - ( It will look Stupid.) -  Now export that S4S Mesh and Open in Blender.  You will see that the Object that you are Swapping out looks Horrible, more than likely', becuase it has your Custom Map.  Now, IMPORT those .OBJ files and make sure they have the Correct Material Block checked.  You will notice, that because you Previously' Mapped these already - They should look Perfect.  Now, you can JOIN these Mesh Parts and Delete the Swap-Out S4S Mesh.
The usual:  Make sure the New Mesh has the Same Name and Cut as the one you are replacing it with and just Save to Overwrite the Old S4S Mesh and Import. 

Hi,I want to convert TS3 object to TS4,but I have problem with importing textures.In TSR workshop there are more than one texture for each object,but in S4S you just can import one texture,while I need to import all of them,and when I export textures together,nothing shows up,there's a blank .dds file or .png,can you please tell me how to fix this?

Hi virginia, one thing to keep in mind is that Sims 3 objects and Sims 4 objects have different types of textures based on the fact that in Sims 3 most objects used the in-game CAST system and Sims 4 has multiple fixed diffuse textures instead. Esmeralda wrote about this in her tutorial that goes over how to convert items from Sims 3 to Sims 4 HERE.