Generally your fiance should be able to add you to her insurance policy as a driver on the vehicle. If you two are in the same household the insurance company would most likely require you to be named on her insurance policy. Most insurance companies request all household licensed drivers to be listed.

If you are asking if you can be the primary named insured on the vehicle, then you will have to ask the current insurance company if that is allowable. Insurance companies' guidelines differ, as do state insurance laws. You might decide it is time to shop around for auto insurance for you and your fiance if you are combining car insurance policies. If this is the case start here getting your free car insurance quote.