One possibility would be to create a duplicate of the sprite you want to make glow; make the duplicate a child of the original.
Change the Modulate value for this duplicate to something bright yellow/blue etc., or whatever color you want to make the glow.
Scale it up x & y slightly, like 1.1.
Then turn on the checkbox "Behind parent" so that of your sprite so it will be behind your original.
Next, drop the opacity to between .5 and .8, so it's somewhat translucent.
Finally, change the "Blend Mode" to "Add", which will brighten it up.
(Alternatively you could hand paint the glow you want to use in GIMP etc. and follow the same steps)
At this point, you should see a colored light around the edges of your original sprite.
Now whenever you want the glow, you just turn on the glow-sprite's visibility, and turn off the visibility when the glow should end.
Hope that helps :)