Website Migration

I’m new to WordPress, and I have a client whose website I developed (not a blog, not in wordpress). She has a wordpress blog (not hosted by wordpress). She would like me to change certain pages of her website to work with wordpress software so that she can change them rather than having me do it. If her website host can support wordpress, is it possible to selectively change only certain pages to be in wordpress format, or do I have to change the whole website?