AddImageFile method of FRDocument object contains PageIndices parameter.

HRESULT AddImageFile(
  BSTR               ImageFileName,
  IPrepareImageMode* PrepareMode,
  IIntsCollection*   PageIndices
);

use the following C# code to open only 2nd page of an image:

FRengine.IIntsCollection PageIndices = FREngine.CreateIntsCollection();
PageIndices.Add(1);
FRDocument.AddImageFile("MyImage.pdf", null, PageIndices);

Only the pages with numbers added in the PageIndices collection will be added to the document.