For some reason there seem to be a lot of dogs around with easily upset tummies.
The dry food is probably enough - it is all some dogs ever get. So if that's working for you, why mess with it.
As for adding something or changing to something else, it helps to do it little by little ie only swap over one quarter at a time (or for really sensitive dogs, one spoonful) ie spoonful new mix and one spoonful less old mix each day until its all new mix.
If you want to add something like sardines or yogurt, only put a little bit per meal ie 1/2 a sardine instead of the whole tin. My dog copes with 1/2 a tin but if I give her a whole tin's worth in one serve - things get ugly with the number 2s.
Yogurt - I give a couple of desertspoonfuls with no adverse affect. Carrot, whole carrot bad, half or quarter ok. You willl have to work out for your own dog what works and what doesn't.