is there a way to move the 3D Cursor on top of anything? Many times that I have converted a "View Space" Grease Pencil stroke to a curve, I have found that the curve was created and placed in where the 3D cursor is(not exactly but they have relationship).

However, I don't want the curve be placed inside any object(see fig below),I found no way to control "the deep" that where the curve will be placed.
Set Stroke Placement to Cursor. Then left-click on an object to set the 3D cursor location and the Grease Pencil will be drawn in the plane of the 3D cursor.



If you don't want the 3D cursor to be placed on a surface, you can manually adjust its location in the N panel of the 3D View: