I guess drawing the same thing multiple times at slight offsets with a lower opacity (or darker with addition) wouldn't suffice? Seems a bit of an intensive way to do it
Another option, however, is to draw a lower resolution version scaled up (and smoothed).
You could use either a lower resolution texture or draw the full resolution texture scaled down on a render texture and then draw that scaled back up.
Of course, remember to turn on texture smoothing.
EDIT:
Here's an example of a test I just made for you
:
Here, the right-side sprite is drawn to a render texture at an 1/8th of its size and then scaled back up with texture smoothing and drawn to the left-side.