A good example of a regular Action being sorta used as a Reaction is the "Delay" action. You can find it on page 221 of the Dark Heresy 2nd Ed. Core Rulebook.
Type: Full Action
Instead of acting hastily, the character waits for a better opportunity. When he chooses Delay, his turn immediately ends. At any time before the start of his next turn, the character can perform a Half Action of his choice - even though it is not currently his turn. This later counts as being part of his turn, though. [cont...]
If your psyker has time to know the fire is coming (at least 1 Round's notice) you could pick Delay and then use Manipulate Flame as your action. It would not count as a Reaction under this circumstance. Note, however, that this lets the character keep their Reaction, meaning they can still try to do something else, such as Dodge or Parry.
Would you allow this?
I would probably allow this as a GM: the power in question is incredibly situational and relevant to being shot at with fire. So, it's a clever use of the spell, which should be rewarded, but it's potentially game-breaking to allow psykers to use powers as Reactions. I would also probably explain that this usage is unique and make sure it isn't abused in the future.