we are sorry. You can create subfolders using Rebex EWS, but you cannot specify "special exchange class" for it.
All newly created subfolders will be of the 'IPF.Note' class, which is default.
Sample of creating subfolders:
var ews = new Ews();
var inboxId = ews.GetFolderId(EwsSpecialFolder.Inbox);
var testId = ews.CreateFolder(inboxId, "Test");
var subTestId = ews.CreateFolder(testId, "Test-sub");