Fred,
Are you looking for the Morse 4.5 to 3 adapter sleeve?
The 6900 series lathes all have a Morse 4-1/2 size bore (I think).
The 6900 I have came with an adapter sleeve with MT4.5 on the OD and MT3 on the ID. That gets you to a more standard MT3 for which premade centers are widely available.
I have seen these adapter sleeves from time to time on Ebay. Right now, the only thing I find that is close is a MT4.5 blank that you could turn to a center.
http://www.ebay.com/itm/Morse-Taper...621?pt=LH_DefaultDomain_0&hash=item1c3037c14d
For $25, it might be worth a try. Set up your compound for a 60 deg center and start turning. Looks like the blank is long enough that you could re-make the center several times.
Terry S.