Design note: normal Accessory items use a different skeletal system to other products so the Attachment Tool won’t work correctly in this context – to include a pose meshes must be weighted to the avatar using the clothing skeleton, which give access to individual bones of the avatar. Once this is done the result can be use to pose the avatar.
The Attachment or Accessory skeleton (image top) differs from the normal avatar skeleton as it only contains two specially named bones. As these only work for accessory items, to include poses or other objects, the clothing skeleton (image bottom) has to be used.
When using Append make sure to only bring in objects from the Object folder , no other data is needed – in this context, Append, *.blend files act like zip or rar archives that can be browsed in similar fashion, double-click to drop down/back up the directory structure/folders to see their contents.