Hi bin_go,
I don't think Nikon has ever made an adaptor for Nikon 8400 to Nikon's scope.
For connecting the 8400 to any scopes, I think, you need the Nikon UR-E14 connection tube first, then find an adpoter fitting it to the eyepiece, such as DCA for Swar scope. Our experts Neil and Bob Thomson have given a full details of it at the following thread.
http://www.hkbws.org.hk/BBS/viewthread.php?tid=801
The following thread may be also helpful.
http://www.fuji.com.tw/dscacc.asp?AID=4571
Wing
