I believe there's a solution for that, since the issue could be related to body partitions. Try this:
1. Make sure the SMP weights remain the same after you copy the rigging in OS.
2. Once you finish the OS project, check if nifskope can open the original mesh, and if it looks more or less like the original outfit (sometimes the custom smp skeleton changes, if so, copy the branches from your nif to the scene root in the original mesh and overwrite your mesh with those). Make a backup of your mesh, as it is still useful.
3. Try building your meshes, try them ingame.
4. If the game crashes when you equip it, try importing the conversion mesh into latest blender with latest nif plugin, and export it with a different name. Fix the bsshaderproperty on the output mesh after you export.
5. open your project in OS, check if morphs are working, if so, import the blender output mesh into OS, copy the morphs to the new ones, delete the old ones and save the project
6. Try building your meshes, try them ingame.