Making LOD's for Multiple Mesh Groups

You are not meant to join it with the reference, you need the nude body. But since your garment seems to be of the same exact length than your reference, then you can use it

1). How you would do that normally:
If your dress originally has 3 meshgroups (split in the calf area), clone a swimsuit with no sculpted details. Once you do, Tools - Modding - embed all externally referenced resources. In the warehouse you will be able to tell how many meshgroups per lod there is. You want a swimsuit to be split the same way as the dress in the original package. Export its lod1-3;
How you can do it:
Just export lod1-3 from your dress package.

2). Open the file that contain your garment, File - Save as, name it All lods;
3). Select your mesh, delete all body parts so that only the dress remains;
4). Add Decimate modifier;
5). Duplicate the mesh twice so that you have 3 of them in the scene;
6). Select the first one, rename it to lod1, decimate it, apply the mesh;
7). Do the same for lod2 and lod3;
8). So you have three dresses in the scene how with different names and polycounts. Save the file;
9). Open the blend file with lod1 that you exported from the package. Delete the dress, leaving only body parts, append the decimated dress from the All-lods file that is named lod1;
10). Now you can join the dress with the body. Make sure to select the dress first, then the body, so that you will not have to change cuts;
11). Do the same for lod2-3