There are several ways to use that interface. One of the easiest is just connecting via Browser or another Script that parses html pages like PHP HTTP Client and so on. The ServerAdmin Tool is just only a gui for this Interface on Port 311.
If you enable ServerAdmin´s debug capacities by entering
in your shell, you can see that ServerAdmin uses these XML Files to communicate with the servermgrd daemon on your server. So the XML Files (Dialog) are a dirty workaround to things apple forgot to implement into serveradmin.