It depends on the alcohol already in the mead. Since alcohol is toxic to yeast, the newly pitched yeast will probably die and not do anything if the ABV is 5-7% or above.
When you add yeast initially, the alcohol builds up gradually so that the yeast acclimates to it gradually until it's overwhelmed by the alcohol. Usually, that's at 14% or so, but EC1118 can be pushed up to 18% if it's happy.
However, by just putting in dry yeast into an environment with alcohol already in it, the yeast will probably just die.
A yeast starter can be made, with more of the mead added to it gradually so that it acclimates to it. That's probably the only way to do it.
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