S&S has complete rules for crafting poisons. Rokugan has a number of other poisons (and, actually, different rules for crafting poisons). Basically there is a craft DC and, like all other craft skills, it costs 1/3 of market price for materials. The creation time is ridiculous though. It will probably take most of the week to make a single dose of poison. I'd recommend changing the base time to a day for poison crafting.
UofMDude