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Can't be I don't think.
Contents of a med box are defined through cfg vehicles in the dayz code.
You could probably use something server side like this,
http://opendayz.net/threads/tutorial-remove-weapons-replace-weapons-on-vehicles.11070/#post-51451
Following those instuctions you could probably try
if (_object isKindOf "MedBox0") then {
_object addMagazineCargoGlobal "ItemAntibiotic";
};
I have not tested this but it should work, when the server spawns the medbox it should add the antibiotic.
Try it and see what happens.