Ok, I'm keeping this topic here for the moment just because this might venture in the realm of particle stuff which is listed as belonging to advanced..
Anyways, there're at least two ways you may affect the explosion dynamics.
You can create an addon including new exploding shells with much larger damage area which you then use for your explosion, but it would be like the default explosions without any blast wave (if that's what you're looking for).
Or you can script the blast wave with some vector/velocity/setdamage/whatnot script(s).
Of course you would need to create the mushroom cloud with particles.
I'm sure Mandoble has something to add
*cue*