Best practices of converting an already created site into Craft CMS?
If you're not already on a CMS, I'd recommend migrating to Craft sooner rather than later. Sounds like you're definitely planning on migrating to Craft, so I see no valuable reason to delay.

Pros of migrating now:


Awesome Craft features, like Matrix
Plugins
Twig


Cons of migrating later:


More legacy code to port
More legacy data to restructure


You'll notice that both of those lists reinforce the idea of migrating now. ;)