I am not sure what you mean by drag and drop covered cells.
The \Syncfusion\Essential Studio\3.2.1.0\Windows\Grid.Windows\Samples\Quick Start\GridPad\ sample has a menu item that allows you to drag a selection of cells and then use the menu item to cover them.
To do that, you just add a menu item (maybe with a hot key), and then in the menu handler do something like:
private void menuItem3_Click(object sender, System.EventArgs e)
{
if(!this.gridDataBoundGrid1.Selections.Ranges.ActiveRange.IsEmpty)
{
this.gridDataBoundGrid1.Model.CoveredRanges.Add(this.gridDataBoundGrid1.Selections.Ranges.ActiveRange);
}
}