You can add the Google PageSpeed subdomain by going to your CloudFlare DNS settings. Go to:
Settings->DNS settings->add subdomain->make sure cloud is grey.
Please follow these steps from Google:
1) Follow PageSpeed Service's setup process.
2) Make sure CloudFlare is ENABLED (orange cloud) for your origin domain
3) Make sure CloudFlare is DISABLED (grey cloud) for your www CNAME pointing to Google (the record pointing to pagespeed.googlehosted.com). If you do not do this, then you may see a "DNS Points to Prohibited IP" error visiting www.
With the above configuration, the PSS console continue to say "DNS Changes Required". Despite this message, your site will retain the benefits of both PSS and CloudFlare.