Well, AFAIK, you can just edit the weapon, open the art tab and change the output area there... I'm not on a GECK PC right now but I will be in about an hour, then Ill check it out.
Gotcha: In the weapon screen, lookup the impact data and search for it in the GECK. You will get a screen, "ExplosionImpactDataSet" for missles, which will show which type of explosion will happen on a certain type of ground. Note the name of that type and look it up in the GECK. There you are able to edit the NIF file.