I need to create small auxiliary server that is part/integrates with OpenFire server in order to get roster for a given user.
I was looking for some plugin/api call that given the user JID to return his contacts and their online status.
All-in-all I may end up changing OpenFire's code but I was hoping that there is a easier solution
All though the links provided by Manasi would work for you. However those links refers to user service plugin which has been depricated by openfire.
They recommend to use REST API plugin.
You'll see list of all plugins here.
You should read about retrieving user roster and their presence .