Principal Financial Group Inc., which exited the health insurance business two years ago, has introduced a critical illness insurance product designed to cover expenses not covered by health insurance. The insurance will be marketed to businesses to offer to employees as a supplement to a core employee benefit package.  "Our goal is to create a simple, straightforward benefit employers can offer to their employees," said Amy Friedrich, vice president of Principal's Specialty Benefits Division. Voluntary critical illness insurance provides a cash benefit to pay for additional expenses such as health insurance deductibles and co-payments, travel costs, childcare and home health-care needs. The insurance benefit covers the five most common critical illnesses: cancer, coronary artery bypass graft, heart attack, major organ failure and stroke.