First go to User Preferences->Addons and enable the SelProject addon in
the Mesh category.
It will appear in the Tools panel. First set From and To object from
the dropdown list. From object
is the object you project, To object the one you project on. If
switching to editmode first,
the "Use Selection" option appears. When choosing this you will use a
copy of the selected area
instead of a From object.
Press Start SelProject to start the projection. When in Use Selection
mode, the object selected from
will be temporarily hidden for the duration of the operation. You can
use manipulator and
G, R and S (and XYZ) hotkeys as usual to transform both objects. Also
there is the direction Empty
which is used in combination with the origin of the From object (which will
be temporarily set to
object geometry median) to set the projection direction.
Press ENTER to finalize the selection projection operation.