Is there a way to add a dose of poison to a damage dealing spell? A feat, class option, magic item, another spell?
Not much more to say than asking the question.
I may be wrong but the RAW outlook seems to be that poison is alchemical and spells are mystical. They can simulate each other but don't combine.
The only contradiction I know of is the harm affect of a variant channeling cleric using the poison type. Cool build but feat intensive. If you like that, check these feats:
quick channel (essential)
selective channeling (essential),
improved channeling (recommended)
guided hand,
channel smite
versatile channeling
Consider a dip into fighter at 2nd for add'l feats.
If Aasimar consider Channel Force feat.
Note that extra channel can only be taken once (and should be for an offensive channel build)
There a Str debuff power under the cleric plant decay subdomain that might combo well. Healing, chaos, madness make good second domain choices (though personally I like the protection-defense subdomain for the shield spell so I can uber max AC and still use a reach weapon for improved trip - poisoning is so much easier when they are prone).
In any case, you definitely want to be using poison weapons/claws/whatever to get the stacking affects from this this build. It occurs to me that this build may pair well with alchemist's levels. Particularly the feral mutagen and concentrate poison discoveries, so you can claw/claw/bite with poison claws in heavy armor (from the fighter) while base to base and radiating poison 8 times per day.
This guy runs around radiating stat debuffs and healing or bad touching the enemy for other utility debuffs.