Hello fayed22,
You can put a mesh inside a skin, yes. First you need to create a new skin in the skins section, then just select the mesh in the tree view, and create a new skin placeholder.
More about skins: Skins - Spine User Guide
Regarding your second question, you can duplicate a mesh and not make it a skin placeholder, just click the duplicate button, whihc is next to the linked mesh button. Once duplicated you can replace the path of the image to have a different image set, or you can simply deselect the "mesh" checkbox so that the mesh will stop being a mesh and go back to being an attachment.
More about meshes: Meshes - Spine User Guide
I hope this answers your questions (: