It doesn’t matter how you have your website organized at the moment, as you can still use it to build your list by making your main page, and any others that people arrive at when they find you, into an opt in page.
This way you get most value to your business in terms of traffic.
You might have an opt in page on or off your site but a lot of people do not sign up on it. However, they still want to see what you have to offer. They then put in your domain name to search and get into your site that way. With the opt in page on your main page and any others you get traffic to as an entry point to your site, visitors have to opt in to get access to your site’s niche information.
Make an opt in page.
This is simply a page which has only one intention: to have visitors subscribe to your list. You have no other links on the opt in page which would just take people away.
This gives you an obvious advantage. People visit other sites and can get plenty of information for free. But your site is the opposite. They must sign up to get your information. In addition, when you have whet their appetite for more information from you by previously providing valuable information before they arrive at the opt in page, they feel more compelled to opt in. They have no other choice.
As said, any other links on the opt in page divert people away so you lose that effect. Also, when you stop just anyone accessing your information it gives exclusivity to it: they have to “pay” for it with their name and email address.