In order to connect to a certain computer inside a nat-ed lan you would need to forward all needed ports to that internal IP address. Since ICMP doesn't use anyone certain port you can't forward anything. Only thing you can do is to enable ICMP requests on the router itself. This will reply to internet pings, which is actually a security risk.
As for pinging a certain computer inside. I don't think you can unless you're using a vpn.