I used to use my KA6 mk1 with external mixers all the time
get these cables first:
edit get two sets of the cables, you need left right for deck 1 and left right for deck 2 and those cables are sending a mono signal so each output needs two cables
they will allow you to connect your KA6 out 1/2 and out 3/4 to RCA outputs from 1/4 inch
those outputs can go into the channel one and two of your mixer or whichever way you prefer, basically choose two channels in the mixer.
*edit connect it to the "line" ports on your mixer rear not phono*
Then you can open Traktor and select the KA6 as your interface and then deck a outputs output 1 and 2 then deck b is 3 and 4
Set mixer mode to external and now when you DJ, your sound will go straight to the mixer and you can control the songs that way, but you need a controller or something to cue up songs and play them or pause, cos the computer is still handling the play and pause controls so you may be using your laptop to cue up songs and sync them up
but hopefully that should allow you to use your behringer mixer with traktor!