We use flavoring ingredients like herbs and berries during the initial steep. We don't add anything after the fermentation. We use mint in one of our recipes, and it works well. The advice I would give you is that some ingredients can hamper bacterial growth (for example, we've found that ginger can slow down the fermentation a bit, and rosemary as well) and some change flavor during fermentation (e.g. hibiscus turns very bitter, but rosebuds turn more fruity).
We've not had any issues with oils going rancid in the SCOBY as witek suggests, but we also pull our SCOBY every batch and only use rich starter. When we were brewing in small batches, however, we would use our SCOBY for 3 brews or so, and didn't see that effect then either.