Once your fiancè[e] enters the U.S., marriage must occur within 90 days of his/her arrival. Unfortunately, you may encounter hardship enrolling your fiancè[e] into your domestic health insurance plan (mostly provided to you by your employer), until marriage and in most cases for a certain duration after marriage.
Also, as a newly arrived person in the U.S., your fiancè[e] would not be eligible for regular domestic health insurance. The majority of companies require the person to be a resident of the U.S. for 6 months, 12 months or 2 years at the most. Many companies also require that the person either be a U.S. citizen or lawful permanent resident. And your fiancè[e] would not be able to satisfy those requirements. Moreover, even the eligible people can enroll into regular domestic health insurance only during open enrollment period (such as Nov. 1, 2015 to Jan 31, 2016) and not at any time throughout the year.
As an interim solution, we have excellent insurance products available exactly for the persons like your fiancè[e].
You can consider purchasing International Major Medical Plan. It does not participate in any PPO network. It is not renewable but you can make month by month payment automatically.