It's cool and awesome you're improving your PHP skills - a CMS has it all, including session, database, plugin/module, file manager, editor, layout controls and so forth - great way to learn.
But, take what others said here very seriously. I'm going to expand on previous comments a bit. Learning from a popular and well written framework teaches you good habits as well as saving you valuable time. Beyond learning, or for the fun of it, in real work projects if you've determined there is more work in conversion of legacy code/content than the entire CMS framework, so be it, but be diligent during the research and be sure. I've found that most of the time it's more time and resource efficient to adapt legacy apps/pages - it might involve extracting key data to XML or database formats used by the CMS, and careful testing of revised code. But the goal is to not solely to support legacy stuff, but also to ensure the entire site (front and back end) is scalable and supports modern standards. The client uses the CMS, so it's crucial to this goal.
On smaller projects this isn't so important, but since you mentioned you're learning, thought I'd take a moment to mention this in context of "been there, done that". My personal .02, of course.
-jim