[DarkDeviL]

public void OnPluginStart()
{
        RegAdminCmd("sm_search_steamid", Command_Search_SteamID, ADMFLAG_ROOT, "Search for Steam ID");
}
public Action Command_Search_SteamID(int client, int args)
{
        if (args < 1)
        {
                ReplyToCommand(client, "[SM] Usage: sm_search_steamid <steamid>");
                return Plugin_Handled;
        }

        char search_for_steamid[32];
        GetCmdArgString(search_for_steamid, sizeof(search_for_steamid));

        ReplyToCommand(client, "Searching for '%s', ...", search_for_steamid);

        for(int i = 1; i <= MaxClients; i++)
        {
                if (IsClientInGame(i) && !IsFakeClient(i))
                {
                        char steamid[32];
                        char player_name[32];
                        if ((GetClientAuthId(i, AuthId_SteamID64, steamid, sizeof(steamid)) && StrEqual(steamid, search_for_steamid) ||
                                GetClientAuthId(i, AuthId_Steam2, steamid, sizeof(steamid)) && StrEqual(steamid, search_for_steamid) ||
                                GetClientAuthId(i, AuthId_Steam3, steamid, sizeof(steamid)) && StrEqual(steamid, search_for_steamid)) &&
                                GetClientName(i, player_name, sizeof(player_name)))
                        {
                                ReplyToCommand(client, "Player name for Steam ID '%s' is: '%s'", search_for_steamid, player_name);
                        }
                }
        }

        return Plugin_Handled;
}

sm_search_steamid 76561197961798171
Searching for '76561197961798171', ...
Player name for Steam ID '76561197961798171' is: 'DarkDeviL'
sm_search_steamid STEAM_0:1:766221
Searching for 'STEAM_0:1:766221', ...
Player name for Steam ID 'STEAM_0:1:766221' is: 'DarkDeviL'
sm_search_steamid [U:1:1532443]
Searching for '[U:1:1532443]', ...
Player name for Steam ID '[U:1:1532443]' is: 'DarkDeviL'

Something like that?