Game: 6878 8159, Ohio.
Turn 16.
Played Kiln, then Imp (I had an extra action), and got the 2 cards for Imp. But, no chance to gain the extra Imp, via Kiln. Can't see anything in the text of Kiln that would explain why this would be an exception. Kiln worked as expected [in terms of gaining cards] for all my other plays that game.
The "mantra" for this is
"A gain that doesn't specifically say where to gain from (or names the card explicitly) can only gain from a Supply pile."
So you can't gain an Imp via Workshop, Remodel, Kiln, ...
You can gain it via Exorcist "... from one of the Spirit piles", or Tormentor "... gain an Imp from its pile".
If you were talking about "Ghost", you could also gain it via Graverobber (because it allows you to gain a card from $3 to $6 from the trash), ditto for Rogue.