Official comment
You sure can - just open the DJM-750MK2 Settings Utility and check to see which input is set for the REC OUT:

Then within Traktor's preferences, your Mixing Mode on the Output Routing page should be set to External, and then on the Input Routing page, assign one of the decks to the inputs which match your REC OUT channel (in the image above, it's set to 9/10). I would map mine to Deck D's inputs.
Finally, on the Mix Recorder page, the source should be set to External and the External Input should be Input Deck D.
That's it!