I think it's possible. I have never tried this but I believe that if you agitate in violent bursts of very short and quick jerks and do it during the whole fixing period that you will over agitate.
On a serious note, I don't pay much attention to agitation during fixing. I do a bit in the beginning (say 20ish seconds, not more) and then here and there when the mood strikes me, and sometimes it doesn't strike me at all. Never seen any difference. I am pretty sure you are safe, so long as you are a sane person.