anim8or does not yet have this capability. however, it is a very easy effect to make using post processing with the gimp.
render your image in anim8or first. import it into the gimp. create a duplicate layer of the image (you should be able to figure out how, the button will be in the layer dialog). convert the top layer to grayscale using the desaturate command in the "colors" menu. then in the layers menu (not to be confused with the layers dialog. a dialog is a separate window, a menu is that writing at the top of the image window) select transparency, and then select color to alpha. pick black as the color.
you should now see part of your image in color again, as you have "cut out" the dark parts of your layer, and the layer underneath it is showing through.
now in the filters menu, add a nice blur (using the option that lets you control the exact amount of blur.
now back in the layer dialog, adjust the transparency of this image. so you can see more of the layer under it. you should see a bloom effect now. play around with the settings to get it just how you want it.
you can optionally skip the desaturation step for a different, but still believable effect.
