The first thing that you can try is getting some mouse bedding from your pet store and put a handful in the front of her enclosure on feeding day. When you see her actively tongue flicking and looking for the mouse, drop in a white rat.

The second thing you can do - if she has good weight on her, and isn't a baby is to employ the "tough love" technique. Keep offering her a rat one day a week. If she refuses, don't give in and give her a mouse. At some point, she'll decide that perhaps a rat is a pretty good option afterall.

Of course, if she's starts losing weight (more than a small amount) and you're concerned about her, don't starve her, and offer her mice again. You'll just have to start the "tough love" clock over again.

Most of my founding females were raised just on mice, and this is how I coverted them all to rats when I started breeding my own rats (I had no desire to breed mice).