You could still keep the Bohemia account integration, but make it optional and the Steam credentials obligatory instead so you could gain the benefits of both 100 % coverage of Steam account of Arma 3 users and extra services of Bohemia account.
Steam API documentation: https://steamcommunity.com/dev?l=english
You can even require Arma 3 ownership of authenticated account with the API request functionality! https://developer.valvesoftware.com/wiki/Steam_Web_API#GetPlayerSummaries_.28v0001.29