I'm not sure you can do this with sprites - but you should be able to do that with 3D objects, plains etc. Perhaps switching from sprites to plains would open up other avenues, and make it worthwhile. With plains, you could use the ghost object command, which should provide the look your going for. I consider this stuff a lot, here are some potential benefits...
* Ghosting - so you get an additive blend, perfect for bright particle effects, lighting etc.
* Negative ghosting - pretty decent for shadowing, as ghosting tends to have less issues than transparent objects.
* Resolution independance.
* Shaders, should you ever need them.
* Animation - like a simple bone structure on a segmented plain - I've used this for bendy spines and snakes and all sorts.
* Extra cameras, for maps or whatever.
* Performance - plains should be faster than sprites.
* Z-Depth - usually easier to arrange your z-depth with 3D plains.
One thing I have found, is that particle texture generators don't often support alpha - they concentrate on black backdrops. These textures are practically impossible to convert to alpha, there are just so many factors, and the black backdrop particles with ghosted plains look so much better than trying to do it with alpha and sprites.
Alpha transparency is good for things like smoke effects, effects that are not additive - like lots of smoke particles will be grey, but lots of fire particles should be bright - as they overlap they should get brighter and brighter. Sprites can only really do the smoke, wheras plains can do both.
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