A roster isn't a global list for all users. A roster is the user's personal list of contacts. This list is stored at the server, so it will be the same regardless of which client the user uses to connect to Jabber.
The Jabber SDK (CAXL) also gives you the ability to add quick contacts (a list of contacts available for the session). For example, you can write a Jabber web application to add Finesse contacts as quick contacts.
You can use CAXL to get presence information for a given contact in your permanent roster, or presence of a temporary quick contact. How you display it is up to your programming ingenuity.
Quick Contacts is a Cisco feature - not part of the XMPP specification.