public class MyGenerator : IIdentifierGenerator
    {
        // TODO вынести из хард кода:
        private const int OrgId = 219;

        protected string TableName;

        public object Generate(ISessionImplementor session, object obj)
        {
            var connection = session.Connection;

            long seqNextValue = 0;

            ConnectionState originalState = connection.State;
            if (originalState != ConnectionState.Open)
                connection.Open();
            try
            {
                IDbCommand command = connection.CreateCommand();
                //TODO заполнять как-то TableName и строку сделать такой: "select nextval('{0}_id_seq')"
                command.CommandText = string.Format("select nextval('role_id_seq')", TableName);
                seqNextValue = (long)command.ExecuteScalar();
            }
            finally
            {
                // Close the connection if that's how we got it
                if (originalState == ConnectionState.Closed)
                    connection.Close();
            }
            return seqNextValue * 1000 + OrgId;
        }
    }

    public abstract class BaseMap<T> : ClassMap<T>
        where T : DataAccess.Entity.BaseEntity
    {
        protected string tableName;

        protected BaseMap()
        {
            Id(x => x.Id).GeneratedBy.Custom <MyGenerator>();
.....

/// <summary>
    /// NHibernate генератор ID
    /// </summary>
    public class MyGenerator : IIdentifierGenerator
    {
        /// <summary>
        /// Id орагнизации из БД
        /// </summary>
        private long _orgId;
        /// <summary>
        /// Следующее значение последовательности из БД
        /// </summary>
        private long _seqNextValue;

        /// <summary>
        /// Генерируем id для вставки записи в таблицу. 
        /// Запросы написаны для PostgreSQL. 
        /// Для другой СУБД нужно писать развилки.
        /// </summary>
        /// <param name="session"></param>
        /// <param name="obj"></param>
        /// <returns></returns>
        public object Generate(ISessionImplementor session, object obj)
        {
            var connection = session.Connection;

            ConnectionState originalState = connection.State;
            if (originalState != ConnectionState.Open)
                connection.Open();
            try
            {
                IDbCommand command = connection.CreateCommand();
                command.CommandText = string.Format("select nextval('HIBERNATE_SEQUENCE')");
                _seqNextValue = (long)command.ExecuteScalar();

                command.CommandText = string.Format("select CAST(value AS bigint) from setting set WHERE upper(set.name) = upper('org_id')");
                _orgId = (long)command.ExecuteScalar();
            }
            finally
            {
                // Close the connection if that's how we got it
                if (originalState == ConnectionState.Closed)
                    connection.Close();
            }
            return _seqNextValue * 1000 + _orgId;
        }
    }

    public abstract class BaseMap<T> : ClassMap<T>
        where T : DataAccess.Entity.BaseEntity
    {
        protected string TableName;

        protected BaseMap()
        {
            Id(x => x.Id).GeneratedBy.Custom<MyGenerator>();
......