Is it okay to mix...?

Yep, that would be the best way to transition. Just make sure you're using food specifically for puppies. In a few weeks (at about 6 weeks old), I would start making the gruel with some water, dry puppy food, and wet puppy food. It should be about the consistency of baby food to start, then you can gradually make it drier until about 8 weeks, at which time just dry and wet mixed should be fine. Good lucky with your puppy! They're so sweet at that age!