This is done with the netscape.security.PriviligeManager . If you have a correctly signed applet, then the Manager will display a dialog box to the user, asking them if they would like to grant this request to you. This happens only once for every privilege that you ask for, but if you restart netscape, it will ask again. There are many Privilege's. The code for priv.java given below shows an example of you to access System properties that are not normally available.
Most people wants to do it for free.
And last time I looked at the newsgroups and mailing groups, every second applet signing question was about signing applets. I have some pointers you want to do it yourself . Get an object signing certificate. You need Zigbert or Zigship. View the java Console to see the output.