This could be interpreted two ways, so I'll try to answer both..
If you want to add the part's hidden lines, click on the view/views that you want to show hidden lines, in the navigation pane under "Display Style" click "Hidden Lines Visible"
Or you could be asking how to INSERT a hidden or phantom line.. First go to sketch, draw your line(s). Now right click in the grey part of the Command Manager and add the "Line Format" toolbar. Pick the line(s) you would like to make Phantom. Click the "Line Styles" Icon in the Line Format Toolbar and choose phantom.
Hope this helps.