I don't see any problem with pulling the eggs out from under a hen on day 18 (the day you're supposed to stop turning if they were in the incubator the whole time) and putting them in an incubator to hatch.
Another thing you may want to consider is splitting the fertile eggs between the broody and the incubator, for a couple of reasons:
1. Your chances are better that some will hatch. If your broody isn't good at her job or if the incubator setting are off, it's better to have both bases covered.
2. If you leave your broody raise a couple of chicks, she will be happier, may not go broody as often, and there's really nothing better than watching a mamma hen with her chicks. You can keep a couple of chicks inside too to bond with your family.