It's actually possible with AttachCell. Try:
FrontEndExecute@
 FrontEnd`AttachCell[
  EvaluationNotebook[],
  Cell["asdasdas", "DockedCell",
   CellFrame -> {{0, 0}, {0, 1}},
   CellSize -> {Scaled[1], Automatic},
   Background -> GrayLevel[.98]
   ],
  {0, {Left, Bottom}},
  {Left, Bottom},
  "ClosingActions" -> {}
  ]
That will persist until you close the notebook