Single phase hook up to 3 phase box
Single phase power is just one (or two) poles of a 3 phase system, ALWAYS. So no, there is no "safety" issue, it is done all the time. The only issue is that it MIGHT be that the heating elements inside of the coffee brewer are configured to be connected Line-to-Line, instead of Line-to-Neutral. If so, then they will get 208V instead of 240V. Since the elements have a fixed resistance, the wattage output of the elements will drop to 75% of what they are rated for at 240V (P = V squared / R). That then will mean that it will just take longer for it to heat up the water.