Had a player ask about this before and thought it was a pretty good idea.

If a pyromancer is sustating Sculpt Flame already and an enemy shot a flamer at him could he spend his reaction to make some kind of willpower based test (i dont know what against yet) to "parry" the flamer attack.

Sounds like an awesome, flavorful use of a normally-weak-in-combat power. I'd make it a Very Hard (-30) Willpower test, as the power isn't explicitly designed to do that. Alternatively, make it a Challenging (+0) test, but do not allow an agility test to avoid the damage (though allow the ag test to avoid being set on fire, for balance).

was actually thinking on maybe a -20 will power(like most other not using something you know tests aka wielding a gun untrained) and if the get 3+ DoS maybe require the flame weapons user a Agi/Dodge test to avoid getting hit by his own weapon.

If you want it potentially reflective, increasing the difficulty is a good idea.