I know of a proxy that isn't listed, can I add it?
Yes, you are more than welcome!
Just visit the add page and enter the proxy's IP (or hostname) and port. It might take awhile for the proxy you submit to show up in the list.
After submitting an IP and port, you will not get any sort of indication as to whether or not that host is an open proxy.
If you want to scan, do it with your own resources and from your own connection.