The item looks much different in the game than in S4S

I think what you are seeing is a combination of slightly iffy normals, and the edges of the texture being too close to the UVmap, plus some black dots in the texture. The UVmap issue creates small white/black dots, while the normals issue creates sharp lines in what you'd want to be a smooth surface.

The UV issue can be fixed by adding some texture in the edges and by drawing over the black dots/lines along the edges in the texture "isles", to cover the UVs a bit better. Make sure there's at least 4-5+ pixels of matching texture around each standalone "isle" of UVs.

The normals issue can often be fixed by selecting the pieces you want to keep smooth, and choosing "Remove Doubles". If you want any of the parts to keep hard edges, you select those edges, do Ctrl+E and "Edge Split" (make sure to select a full loop if you want to create a full separation, or you'll get items that hang back). The UVs should keep as-is if you've already UVmapped.

It's easier to see and edit the normals if you turn off the texture (not sure how the new Blender looks, I'm still using 2.76, but in the old one it's the sphere drop-up box called "Viewport shading" at the bottom).

Merging the object with the original TS4 one isn't necessary - you make the object, then change the Cut number to whatever the sim object was. Do this for shadow groups if necessary, too. This should be enough to replace the item when you import the file into Sims4Studio.