If you have a drawing program like PaintShop Pro, you could draw a symbol or lines and save them with a transparent background and add the drawing to the video overlay track. With zoom/Pan you could pick out the symbol or line of interest and then move it to the desired location.
I'm a bit new at this so perhaps some others will have a better idea.
Or you can use the chromakey filter to drop-out the background. I just tried it with my Ulead PhotoImpact. I first saved a snapshot of a frame, placed it as an object, and drew my arrow with the "line and arrow tool." Then I deleted the picture and saved the picture and chromakeyed out the white. It worked well, but you're not going to mark up the screen Don Madden style like that.
Plus, if your object is moving, it would make iit a little harder to follow with the pan without keyframes.