Hello.
1) This is the info from the magic.cfg:
Code:
DamageType = None
; For shots that can damage surrounding area - hit type, range, damage and blow.
; Hit type controls which things will be affected by the spell (1..4).
; The damage and blow will be at its max only very near to the casting point;
; at distance it will start decaying, until zero is reached at given range.
; Blow is the strength of hit wave which pushes creatures on explosion.
; Range is in subtiles, damage is in points per turn.
; Note that range and damage are increased with caster exp level.
2) Easy answer: You can't. Hard answer: By modifying the source code of the game. The setting is currently hardcoded.
3) You cannot add a spell, but you can replace another one. Say you don't like the firebomb spell, you can change it into a second fireball spell by changing the last line of the instance in creature.cfg. If you are using version 0.4.6u you can replace spells that are currently not used, for example the spell 'CAST_SPELL_TIME_BOMB' and assigning that to a creature. You can accomplish exactly what you want with this.