I don't personally feed rabbit food but only SOME rabbit food brands are ok for chins, such as the Manna pro rabbit foods, Purina Show rabbit food and Producer's Pride rabbit food to name a few. You'd need high quality rabbit food if you are going to switch, or mix, not just any old rabbit food, most are not properly balanced for a chinchilla so you'd have to research the ingredients and percentages of nutrients to make sure it's right.
Personally I wouldn't mix pellets, I'm unsure what the point would be since that would probably throw the nutritional balance off. If you are going to switch to a high quality rabbit food instead of chinchilla food, just switch completely, once you've transferred them to the new food slowly over time that is.
If I had more chins I might consider switching to rabbit food since cost per pound is cheaper on the large bags of food, but for me the cost isn't too bad with currently only having two chins.