I'm not sure, but don't think they can be crafted.
??? That is really strange. Ive tried with an oil barrel (thought it would be logical) but that didnt work. I really need those fire arrows to light my Midnight oil since I dont have the right magicians in my party (that is, the same caster that can cast the midnight oil is also my pyromancer).
Who is behind the save game editor? Is it Larian or a community member? Would really love to see something like that ASAP.