I wouldn't suggest it. Seeds and other such foods in hamster/gerbil foods are high in fat and aren't really good enough to fully support a rat. Are you unable to find good food in your area or is it a budget issue? If finding the food is an issue I will gladly help you track down a decent food. If it's budget I'm still happy to help as I know Oxbow RR can be pretty expensive and so can Harland Teklad. Feel free to message me if you would like some help! :3
We won't be going to the Pet Store or any place that carries Rat Food until friday, and so I would like to make sure Missy doesn't starve. If I picked out absolutely all of the nuts and seeds would it be safe?
It would be better than nothing to leave some of the less fatty seeds and maybe nuts (I don't know them off the top of my head, but you could probably search them up and find out which ones are okay) and then supplement with some tablescraps until Friday. Meats and veggies to keep her healthy if you can. Babyfood would probably be the best supplement if you could give her some of that, but you might not have it handy. ^-^