Cool, but make sure this is the ONLY explosive you'll use, because it'll edit all flak explosions.
1. Go to
FPS Creator\Files\gamecore\debris directory.
2. There you shall find two files:
fire.dds &
smoke.dds.
3. Edit these to change the explosion, meaning the
fire file changes the look of explosions and the
smoke file changes the look of the smoke.
4. Again, I don't fully understand what you may want, so just edit those files and you should come out fine.
That is the only way to do it, I'm afraid, though I'd
LOVE to be proved wrong.