Not something I'd try to do, personally.
Make sure you get a decent gauge of helmet, 14 is preferable, at 2mm thick, but if they use a decent carbon steel then 16/1.5mm will do you fine.
Consider putting some padding in the top as well, as I don't know if you'll be wearing arming caps.
Though if your group does vikings they might avoid doing head blows so they can have things like leather helmets, or no helmets at all.