There are two good methods in LotusScript, either of which should do what you want to do, including copying the attachments:
CopyToDatabase and AppendAsRTItem
Set newNotesDocument = notesDocument.CopyToDatabase( notesDatabase )
This method will create a new document based on the current document. You can specify the current database as the "new" database location, and the current document as the original document. Your result will be a new document, in the current database, which should be identical (except for it's UNID and NoteID and modification date/times).
Call notesRichTextItem.AppendRTItem( notesRichTextItem2 )
This method will take a rich text item and append it to a rich text item in another document. You have to already have created the 'new' document and have a handle to the "new" Rich Text field. Here the notesRichTextItem2 is the "new" field.