Can I put a painted stop line at a yield sign location?
Yes, you can put a line to show where the approaching vehicle should yield, but the marking is required to be a yield line, not a stop line. A yield line is a series of triangles facing traffic as shown in the pictures and figure below. As with a stop line, a yield line should be perpendicular to the lane it is in (extending across the lane it applies to, as shown in the figures below), not parallel to the cross street. At a railroad grade crossing with a Yield sign, the line may be a yield line or a stop line.
Traffic Sign Handbook for Local Roads, 2011 New York State Edition, September 2011 (pdf) - Chapter 10 (page 93)