[Xymon] xymon and monitoring hosts via port forwarding

I have a couple of smaller sites where our connectivity to them is only via port forwarding over a small business router. No VPN tunnel setup. Basically there are a couple of computers there that we'd like to monitor.

Can xymon receive messages from a host via port forwarding? And what would the entry in the hosts file look like for such a setup? Can you give an IP address and port for it to communicate back the other way, i.e. pings, etc?