The MessageClass won't show up, nor will any others such as "IPM.Note". The
items you create will be PostItem objects that have that specific
MessageClass.

Assuming that your Outlook.Application object is olApp:

Dim oRSS As Outlook.PostItem = olApp.CreateItem(OlItemType.olPostItem)
oRSS.MessageClass = "IPM.Post.Rss"

Then you can do whatever you want with the item such as populating it and
moving it to a specific folder, just as you would with any other Post item.

That item has the same MessageClass as an RSS item that is delivered to your
store, but it won't necessarily have the same properties as other RSS items
and won't have been received so it won't have the various sending properties
filled in.