Yes. Allow me to explain.
First off, this is in no way to contradict dpatchery. I do agree though to some extent with what he says, but there was something else about it that came to mind and this was involving sinemething slightly similar which is the "legendary combo" use of Frost Weapon + Lasting Frost.
How I'm told by long discussion with support:
D&D Specific Rules beat General Rules.
Let me translate all the talk I had with them into something easier.
In the case of the legendary combo, When you use the free action at-will to make all damage from the weapon cold damage, you are both removing any other element the power had and adding the cold keyword into it. As an example:
A weapon power with Lightning and Thunder keywords that would give 1[W] Lightning and Thunder damage on the hit gets those keywords removed and you are adding Cold to the keyword while reading the hit: 1[W] Cold damage).
So we get back to the question you mention.
Lets use the Level 17 wizard encounter power: Furnace of Sands. This power has a zone keyword. Because it already has it, Pyromancy Master does NOT apply because it specifically says
When you use an area or close arcane fire attack power that is not already a zone, the area of the attack becomes a zone that lasts until the end of your next turn. Creatures that end their turns within the zone take fire damage equal to your Constitution modifier.
So this can't apply. Also, note how Furnace of Sands shows its effect line.
Effect: The burst creates a zone of red-hot swirling sand that lasts until the end of your next turn. All creatures are blinded while they are within the zone, and the area is lightly obscured. Each creature that enters the zone takes 1d10 + your Intelligence modifier fire damage.
We will see in a moment how this relates if you still don't where I'm going with the class feature.
Now lets take the Level 5 Wizard Daily power: Fireball. This power does NOT have the zone keyword. Thus the rest of the class feature functions. The fireball power now has the Zone keyword only because the pyromancy master was specific to a fire attack. Basically, the ending power would look something more like this:
Hit: 4d6 + Intelligence modifier fire damage.
Miss: Half damage.
Effect: The area of the attack becomes a zone that lasts until the end of your next turn. Creatures that end their turns within the zone take fire damage equal to your Constitution modifier.
If the effect line from Furnace of Sands was instead the Class Feature for Pyromancy Master, it would basically do the exact same thing replacing the imaginary effect line of fireball with the one from Furnace of Sands.
Ultimately though, it comes down to DM's final word.