You can do a border run in the last day(s) of your visa and you'll get another 90 days. However. If you leave the country within those 90 days and come back in it will be no longer on that visum terms.
I have to add to be correct: this can only be done on a one yr ME non-B from a consulate outside the country not on an extension of stay.
served me and lot's of people I personally know very well over the years. Is always involved and goes the extra mile if you run into difficulties. Not just a paper pusher but also a (legal) fixer!