I don't know of a built-in way. You could make a button to do it. Something like the following:
Button["gridify",
With[{mat = MakeExpression[
NotebookRead[EvaluationNotebook[]],
StandardForm]},
If[MatrixQ[Thread /@ Thread[mat]],
NotebookWrite[
EvaluationNotebook[],
GridBox @@ Replace[
mat,
_ :> "\[Placeholder]",
{3}
]
],
Message[General::matrix, "selection", 1]
]
]
]
It converts a selection to the grid of placeholders, if the selection is a matrix. You could put such a function into a palette.
Another way is to create the grid from a matrix, fill it in, and paste it where you want:
mat = {{a, b}};
CellPrint[Cell[BoxData[GridBox[Map["\[Placeholder]" &, mat, {2}]]], "Input"]]
Finally, do you know about control-comma and control-return? They create the type of grid you want. I find it easy to hold the control key down and hit comma or return until I have a matrix of the right dimensions. If I overshoot, I can select a row or column and hit delete.
In terms of how I like to use Mathematica, my three suggestions go from worst (but cleverest?) to best, for some reason. The only reason I can think for using one of the cleverer approaches is to create a user-interface in which I want to create an input grid that depends on user input. Otherwise, control-comma/return is what I actually do all the time.
