My shoes get distorded when i add weight to my sims in CAS

Hi, I think that is a uv_1 problem if it's related to body weight morphing.
Your shoes uv_1 is not where the feet need to be, check maxis object to compare the uv_1.

Hi. Joining your shoes with maxis ones does not transfer the data, they are still non-functional.
- delete s4studio_mesh_2 so that only one meshgroup remains. Shoes do not function like hair, this not how it should be;
- in uv editor enable sync, press B, select the shoes' uvs, P - Selection to separate;

- select the feet, in 3d view press Shift D, then Esc to duplicate them;
- expand the rig, make bottom selectable. Select it, duplicate it;
- select bottom.001, then s4studio_mesh_1.002, Ctrl J to join, rename to Reference;
- select the shoes again, add Data transfer, choose Reference as source object, set the rest like in the pic, apply;

- delete the reference, join the shoes with the feet, give them cut 0000;
- clone yfShoes_PumpsHighOpen and import your blend there. Do not use the old package, it is structured differently from what you need. For the future, check how the mesh behaves in-game before making textures and swatches;