How can i seperate boots from full costume?

The instructions below aren't complete but will give you an idea of what you need to do.

To make the boots a separate package:

1) Clone a pair of boots and export its blend. Take note of its cut number.

2) Open the costume blend and delete everything except for the boots, give the boots the same cut number as the cloned boots, then save the file (with a new name, of course). Do this for each LOD of the costume.

3) Export all of the textures from the costume file (diffuse, bump, specular, and shadow). Then import the boots you just edited and the textures into the cloned boots.

If you simply want to remove the current boots from the costume mesh and replace them with your own boots meshh:

1) Delete the current boots and append your boots into the scene. You will need to adjust the meshes to make everything line up well.

2) If your boots HAVE NOT been weight painted. Unfold the rig and make the bottom, bottom_2, and the feet unhidden and selectable (the little arrow has been checked next to them). Shift + D to duplicate each, then Ctrl + J to join them. Transfer the weights from these body parts to your boots (ONLY to your boots).

2A) If your boots HAVE been weight painted the weights may not be correct. In that case, you'll have to delete all existing weights before transferring new weights.

3) Transfer the uv_1 using the same rig body parts you just used as a reference.

4) Vertex paint your boots 00FF00.

5) Assign a cut number of GEOM 0001 to your boots.

6) You'll have to adjust the uv_0 of the mesh and the textures (ALL of the textures. The diffuse, specular, shadow, and bump maps) to accommodate the new boots mesh you introduced.

7) You will need to make the lower LODS of the boots. You can open just the boots in Blender and lower their polycounts for each LOD and then append them to the costume mesh for each LOD.

If you want the costume to be separate from boots entirely so that you can use whatever boots or shoes you want with it:

1) You have to clone a mesh for your final package that has cuts in the legs. This is so that the outfit can interact with boots and shoes; the package you clone will be tuned in such a way that the cuts in the legs are hidden when boots or shoes are equipped.

2) Your mesh will eventually have to contain an entire sim body from neck to ankles, including the hands. If you delete the boots from the costume you will have to supply the calves and attach the calves to the rest of the mesh.

3) You'll need to separate parts of the completed mesh to match whatever you cloned and you'll need to assign cut numbers to match what you've cloned.

4) You don't have any texture for a sim's calves. This will need to be addressed.

5) The costume blend contains foot bones and if you've removed the boots and attached calves those bones will not be needed. You'll need to remove the bones.

because, when you will delete the boots, there will be nothing left. No legs, no textures. You have to either replace the old boots with the new ones or add legs and create textures for them yourself