Can I add this into my baby's porridge? My mum has asked me to do it to make it more tasty.

Marmite is very high in its salt content and I personally would not give to my son under 1 year old. Instead, you can mix in powdered ikan bilis or chicken stock to make it tastier. They are much healthier and easy to prepare. You can make them in bulk and freeze so that you'll have enough supply for a few weeks. Here is the recipe that I plan to follow once my son is having porridge. http://wholesomebabyfood.momtastic.com/tipstock.htm