Legally: Yes, but reaction of company to carrying excess may vary.
You burn about 3% per hour of extra fuel loaded so, if you stick on an extra 10 tonnes for a 10hr flight you will burn 3 tonnes just to carry it.
I did have a tendency to find reasons to carry a bit extra. I know that this was recorded in company statistics but no one in BA or Cathay ever said a word or challenged my decision.