Have you called GCT office to ask about this as I have so far not seen an optional mentioned? One reason it may be difficult for a tour company to include is that Highclere is only open for tours 70 days per year and tickets must be purchased well in advance. You might do better to make a reservation personally and take a tour as an independent pre or post extension to a planned UK trip. That's what I hope to do sometime in 2014.
It would be great fun to see the "real" Downton Abbey, n'est pas?
p.s. just curious...what tour company did you find that is providing an optional to Highclere?