Recycling and existing circuit

I want to put an outlet in a wall that is entirely a brick fireplace. Obviously that is impractical.
However, there is now a heatilater in the fireplace, that I never use. (If anyone knows of some replacement fans that are not obnoxiously loud; but that is another question entirely.) Can I take the wires that go to the fan and put them to an outlet box? It seems reasonable, but there might be something that forbids it.
At the other fan (there are two in the system) I presumably would have to put in a box also, and then wirenut the wires inside. Does this seem right?
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