It is possible with an old Elan 5000. We tried in the early 90's.
But the background is much higher as in positive mode. So especially for I, Br and Cl we got poorer results than in the positive mode. F we didn't try.
You can choose in the software whether to use positive and negative mode. Additionally the adjustment of the ion lens voltages is split up in knobs for positive and for negative ion mode. The last thing is to convert the polarity of the high voltages at the deflector and the detector.