ionFish is right, you can't transfer those likes, what you want is like forcing all those users who liked your URL to subscribe to your Facebook page, that's technically and legally impossible.
But may I ask why do you need a fan page? how do you use likes on your website? do you use a single URL for all pages? or each page has it's own URL in like button?
By using some of the Facebook tools, you can make your website act like a Facebook page, for example you can use Facebook Comments on your site and when someone post a comment, they will share the story with others.
Anyway, if you want a Facebook page, make it sooner! tell on your website that you have a Facebook page now and invite all your friends to like your fan page too.
Yet, you want to keep that like button on your website to point to your website, for sure you know that each like to your website pages will create story, those likes could bring more visitors to your site than a story created from your Facebook page.
But if your website is not very active and you don't post many new pages each week, it might be better to point your like buttons on your site to your Facebook page, simply by replacing the URK in button's code.