I weaned my pups onto Iams, and other than the very greedy ones, they were never overly keen on it, to be honest. Their Mum has always beent he same with Iams, (Made by Euk) aswell, so I have now changed them.
To add variety for the pups once they were about 6 weeks old, I added a small amount of James Wellbeloved complete sachets. They also has a small amount of minced beef, cooked in water to make a 'gravy' sometimes and went absolutely mad for that. Oh, and they did have a small amount of tinned sardines in tomato sauce a couple of times, (discovered the sardines were the only way of getting Mum to eat her food while pregnant after a tip on here!) and that was their absolute favourite.
I didn't have any problems with upset tummies, and made sure whatever I added was in very small amounts, just enough to give a different smell/flavour.