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