Using the InsertRange method
var fdoc = new ActiveXObject("FileDocument.FileDocument");
var msel = fdoc.CreateEmptySelection();
msel.AddRange(100, 20); // selection: [100..120)
msel.AddRange(150, 20); // selection: [100..120) U [150..170)
msel.InsertRange(130, 10); // selection: [100..120) U [160..180)
msel.InsertRange(0, 20); // selection: [120..140) U [180..200)