How to integrate promotional email template into my magento website without using any extensions? Do we have any alternative method to customize my transitional email to promotional email?
For promotional mails, Magento comes with newsletter functionality by-default. There is also 1 sample template with the stock installation which you can tweak to get your desired templates.
Also, you shouldn't use transaction mails for promotional purposes and you should seriously consider having some 3rd party service like Mailchimp for carrying the burden of all the promotional mails. I'm suggesting this because, there are a lot of technicalities involved with the promotional mails, that you wouldn't want to involve into. You'll need to consider the impacts it can have on your mail server when you are sending a lot of promotional mails per second to your customer base. Many ISPs can block your IP for suspecting a spam burst.