If someone has an steam acc with Heroes VI, then 99,99% is the chance that he will already used the provided key (into his inbox in LIBRARY with the Ubisoft key for Heroes VI) to activate it and link it with his Ubisoft/uPlay account. Once activated, it CAN'T be used anymore even if you have the key, e.g. - useless : )) (as usual). Same as all steam accs who has EA games into (and EA keys be activated again WITHOUT providing the EA accs where the keys were activated ).
So your mission or the thing you're looking for is a fail-story.
My Prices:
Might and Magic Heroes VI Retail Key (for the Retail RU DVD + lang convert files to English): $40;
Might and Magic Heroes VI Steam Gift: $45;
Might and Magic Heroes VI - Deluxe Steam Gift: $50.