That gets tricky if you’re using DynDNS because you have a Dynamic IP address. If so, you can find some solutions if you Google for “dyndns cloudflare”.
Other than that, you’ll need to move all your DNS records over to Cloudflare. When you first add your domain, Cloudflare will try to scan a ton of potential hostnames to see if any of them exist on your current DNS and then copy them over. After that, it will show you what it did copy over and you’ll need to add anything that’s missing.
And then switch your name servers to only the two listed from Cloudflare.
You could delegate your subdomain(s) over to another DNS. I know this comes up from time to time. It’s not the most desirable arrangement, but if you need it, post back and someone can probably point you in the right direction.
Subdomain requests will hopefully refer to subdomains you’ve added to your Cloudflare DNS.
No, you can’t add a secondary server to Cloudflare.