The simple answer is that you should not. The chemistry of stains is proprietary. Stain particles do not dissolve into the glaze melt like other materials, they suspend in the transparent glass to color it. That is why stains are color stable and dependable. In addition, their percentage in the recipe, not the formula, is the predictor of their effect on the fired glaze. Of course they do impose effects on the thermal expansion, melt fluidity, etc., but these must be rationalized by experience and testing. But you can still enter stains into Insight recipes. Consider adding the stains you use to your private materials database (for costing purposes for example).
Ceramic stains are manufactured powders. They are used as an alternative to employing metal oxide powders and have many advantages.
In ceramics, glazes are made by weighing out dry ceramic powdered materials to fill a recipe. Batch recipes often are a combination of a base recipe and additions.
Glaze chemistry is the study of how the oxide chemistry of glazes relates to the way they fire. It accounts for color, surface, hardness, texturem, melting temperature, thermal expansion, etc.