[Lazy]

You can't modify the damage it does but it is possible to fake it by adding more damage after it's been done.
You need to look for the server->client messages that say a grenade is exploding, get the origin and apply damage to people in that radius.
To find out who threw it, do a FindEntityInSphere at the origin and check for grenade entities. The grenade's owner should be in edict_t*->v.owner.

Somewhere on here I posted a function to do damage on players using trigger_hurts, can't find it for some reason but if you can that should work well enough.