Comment 3 for bug 488561

With current Inkscape versions the only way to move elements from the document root into an Inkscape layer is by using the XML Editor. None of the commands from the 'Layer' menu work for elements outside of Inkscape layers (I don't know about the keyboard shortcuts as I don't have those keys on my laptop), and the 'Layers…' dialog only can manage layers not individual elements.

possibly related blueprint:
Better management of layers (duplicate, drag & drop)
<https://blueprints.launchpad.net/inkscape/+spec/better-layer-management>
and its list of related bugs (many are RFEs for a new layer and/or object manager)