Some items are split into multiple parts for a reason. With glasses, it is to give the lenses a shader that enables transparency. With the hair, it is to switch between different shapes of it to prevent clipping with hats. With the outfit you cloned, it is to hide armbands so that they do not clip into the cardigan.
Your outfit must be split into the same number of meshgroups the same way as whatever you cloned. Whatever you clone must function the same as you want our garment to function.
The skirt does not need to interact with shoes or accessories, so it can be joined with the rest of the outfit.
You, i take it, accidentally parented one meshgroup to another, which is why it is "flying" in s4s.
- in the outliner click on the + icon next to s4studio_mesh_1.001, select s4studio_mesh_3, in 3d view press Alt P, and choose Clear parent;
- select s4studio_mesh_1.001, Shift-select s4studio_mesh_1, in 3d view press Ctrl J to join;
- if you want your garment to be in the top category, save the file and re-import it in s4s.
If you want it to be full-body, you need to add legs and clone a full-body outfit to replace with yours. Do not simply change the category or add legs and re-import the file.
The body parts (legs, head, feet) that you are seeing in blender are not imported in s4s.
- click on the + icon next to rig in the outliner, make bottom selectable (cursor), select it, in 3d view press Shift D, then Esc;
- select bottom.001, switch to edit mode (TAB), disable Limit selection to visible, press B to activate box selection, select everything the skirt covers, press Delete, choose Faces;
- select bottom.001, Shift select s4studio_mesh_1, press Ctrl J, save the file;
Normally you'd want to find an item that is meant to function the same way as yours (full-body, affected by skirt bones, excludes armbands). I do not see a single Base game item that meets this description, unfortunately. There is a way of adding meshgroups and making them behave the way you want, but it's not a beginner-friendly task.
People rarely use armbands, so it would not be a big deal to disregard it. The skirt is also barely affected by skirt bones, so you can delete them
- so, select s4studio_mesh_3, Shift select s4studio_mesh_2, then Shift-select s4studio_mesh_1, press Ctrl J. Change cut number to 0000;
- switch to edit mode, select the arm area where those other meshgroups were, Remove doubles with Merge distance set to 0,0001
- make sure the mesh has Armature modifier and rig selected, find skirt vertex groups, delete them, normilize weights
- now you can clone a lingerie set and import your mesh there
