Belief in Ellen White as a prophet is one (number 18) of the fundamental beliefs of the Seventh-day Adventist church.*
The Ellen G White Estate has the following to say:
We believe that Ellen White was inspired by the Holy Spirit and that her writings, the product of that inspiration, are applicable and authoritative, especially to Seventh-day Adventists. … We do not believe that the quality or degree of inspiration in the writings of Ellen White is different from that of Scripture.
The statement “the quality or degree of inspiration in the writings of Ellen White is different from that of Scripture” is one the Ellen G White Estate does not believe. I’ve come across followers of Herbert Armstrong who believe that one day his (and his son’s) writings may be recognised as Scripture. But that’s another story.*Here if the site is down, as it currently seems to be.