There are a few things to consider here. If you want Cloudflare to proxy your traffic and cache any of your content you will need to have Cloudflare set to , which means you’ll also have to be using our SSL. However, you would be able to upload and use your own with our SSL if you were on the BIZ plan.
You can always use your existing SSL to secure traffic between your origin servers and Cloudflare, and let Cloudflare manage the SSL between the edge and your site visitors.
Are there particular concerns you have about using Cloudflare for your SSL?