You'd need a serious set of bioreactors to process faeces ( and non-edible parts of plants, and...) to plant food. The equipment simply isn't present on the station, and given the cost of launching stuff it's probably more efficient to simply lift a suitable nitrate mix to where it's needed than a load of tanks and piping.
Farming in space ( and the associated problem of closed ecosystems) is a terribly complicated and technically challenging issue, and indeed well worth researching.