Q:
I have set up my Xone interface with Traktor as described in the User Manual. The problem is, if I set only the external out within Traktor it does not show the crossfader in the middle.
Is it possible to use the crossfader to mix deck A with B?

A:
Yes. It's possible to set up Xone interfaces for internal as well as external mixer use in Traktor3:

The set-up in the user guide assumes the  4 analogue mixer channels will be assigned to the Traktor decks, typically:
TRAKTOR DECK A = XONE: MIXER CH 2 / DECK B = XONE: MIXER CH 3 / DECK C = XONE: MIXER CH 1 / DECK D = XONE: MIXER CH 4

If you configure Traktor in this way, the mixer crossfader can be used to mix Traktor Decks without using, or needing to map the soft mixer controls for: crossfader, volume, or EQ.
This configuration effectively gives you four, individual mixer channels to use (one for each deck) in the same way as you would if you connect four turntables to any analogue mixer.
The MIDI controls on the left and right of the Xone:4D can then be configured to control FX and other transport, loop and application control parameters etc (or combination of CDJs/turntables etc).

If you only use a couple of Traktor Decks and want to mix between the decks within the software, using the crossfader on the Xone:4D mapped to control Traktor's crossfader via MIDI - this is also possible.
NOTE:
With this configuration, Traktor only provides one main external signal output (that will only be available to one of the Xone:4D soundcard channel inputs).

To set up:
1. In Traktor, go to; PREFERENCES > AUDIO SETUP > OUTPUT ROUTING > MIXER MODE - set this to Internal.
The Traktor crossfader will now be visible in the main window.
2. Set Traktor's MASTER-Left(Mono)/Right outputs to the Xone:4D soundcard channel you want to send Traktor's output to (i.e. OUT 0/1 = SC OUT 1-2 = 4D mixer CH1)
3. In Traktor's MIDI Preferences, go to: PREFERENCES > HOT KEY & MIDI SETUP > MIDI SETUP and, in the MIDI Page GUI, add a MIDI control parameter:
ADD > MASTER > AUDIO > X-FADER
This can be mapped to the Xone:4D crossfader using Traktor's MIDI Learn option, or manually: Channel=GLOBAL, Type=DIRECT, Assign=CC5(Ch16).

NOTE:
To avoid the Xone:4D crossfader affecting other analogue mixer channels when using the crossfader as a MIDI control:
make sure that the Xone:4D's XFADE pot is set to >< and that the Xone:4D's Channel X/Y assign options are switched to OFF, or that the main XFADE switch is set OFF