| Adjoint |
You can attempt to create such a spell, if your GM would allow. Although I would make it transform existing non-lethal damage into the lethal one, not converting the nonlethal damage into lethal as it's being dealt. That way, the creatures that are immune to nonlethal damage are not hurt by the spell, and that's better from the balance point of view; some creatures are designed to be able to ignore certain conditions that would deal nonlethal damage, and if they are suddenly being hurt by them it may beasily become game-breaking.
| Artofregicide |
As I said on your other thread, this kind of spell probably doesn't exist, though I wouldn't put it past 3pp material. But at that point you're in the realm of GM fiat anyway. So why do you need justification for what you're gonna do anyway?
Firebug
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Touch of Mercy and the area version Hymn of Mercy (same link) do pretty much the exact opposite of what you are asking, Lethal to Non-Lethal. So if you were to create your own version, I would say start with that as the base and maybe add 1 level to it (so 6th level) for being non-standard.