How to add a new message template?

/// <summary>
        /// Sends a welcome message to a customer
        /// </summary>
        /// <param name="customer">Customer instance</param>
        /// <param name="languageId">Message language identifier</param>
        /// <returns>Queued email identifier</returns>
        public virtual int SendCustomerWelcomeMessage(Customer customer, int languageId)
        {
            if (customer == null)
                throw new ArgumentNullException("customer");

            var store = _storeContext.CurrentStore;
            languageId = EnsureLanguageIsActive(languageId, store.Id);

            var messageTemplate = GetLocalizedActiveMessageTemplate("Customer.WelcomeMessage", languageId, store.Id);
            if (messageTemplate == null)
                return 0;

            //tokens
            var tokens = new List<Token>();
            _messageTokenProvider.AddStoreTokens(tokens, store);
            _messageTokenProvider.AddCustomerTokens(tokens, customer);

            //event notification
            _eventPublisher.MessageTokensAdded(messageTemplate, tokens);

            var emailAccount = GetEmailAccountOfMessageTemplate(messageTemplate, languageId);
            var toEmail = customer.Email;
            var toName = customer.GetFullName();
            return SendNotification(messageTemplate, emailAccount,
                languageId, tokens, 
                toEmail, toName);
        }