Since it's not apparent whether all your tables have exactly the same structure, the following macro works on the premise that you select the starting cell in whatever table you want to update, then run the macro. The checkbox will be added to all cells below the selected one in that cell's column.
Code:
Sub Demo()
Application.ScreenUpdating = False
Dim c As Long, i As Long, r As Long
With Selection
If .Information(wdWithInTable) = False Then Exit Sub
c = .Cells(1).ColumnIndex: r = .Cells(1).RowIndex
With .Tables(1).Range
For i = 1 To .Cells.Count
With .Cells(i)
If .RowIndex >= r Then
If .ColumnIndex = c Then
With .Range
.InsertBefore " "
.Collapse wdCollapseStart
.ContentControls.Add wdContentControlCheckBox, .Duplicate
End With
End If
End If
End With
Next
End With
End With
Application.ScreenUpdating = True
End Sub
In your attachment, the table has a narrow row at the bottom. As coded, the macro adds a checkbox there, too, which you can delete afterwards. Again, the macro is coded this way because it's not apparent whether all your tables have exactly the same structure.