how can I drag the grid's final row's the width to the right

I’m sorry but your question is not clear. What do you mean under “grid’s final row’s”?

If you want align values in the grid’s cell to the righ you can do that using method mygrid.setColAlign(“right,right,right”);

Starting from dhtmlxgrid 2.1 column resizing not limited by grid’s size. So if you have grid.enableAutoWidth(true) setting - dragging rightmost column to the right will increase width of grid.