I assume you have changed part of clothing outside CC and now imported as Accessory and want to reinstate original weights.

First, the safest way would be to export with FBX key and return character with clothing and weights preserved.
The reason you cannot import back with the key, is because the model got "broken" in third party app (Blender I assume).
There are a lot of reasons for that. Got to be very careful with the avatar bone structure, mesh, naming conventions, vertex groups... etc.
Make sure not to leave any mesh (worse yet curves) unbound from armature before exporting from Blender.

Second, you cannot export weights, but you can import from existed cloth item.
Navigate to the saved CC cloth item (saved before mods outside) and transfer weight from it:




Another workaround I have recently learned about. Say you worked a lot on clothing in Blender, which came with FBX character.
But you cannot re-import that character back to CC because it broke. Happens to me sometimes and I cannot figure what's broken or do not have time to investigate.

Export a nude character from CC fresh with FBX key, open in a new Blender project and then simply Append cloth items from "broken" Blender project.
They are going to come fully rigged and with original weights.
Just export/import character from Blender to CC and you are all set.