Nabble's message composition box offers no way of providing bulleted lists (unless the user is prepared to code their message in HTML).
However, it is possible for an administrator to provide a facility that will produce a result like this:
This is achieved at the cost of losing how an H6 heading is displayed by default. All the administrator needs to do is follow the links at:
Options > Application > Change appearance > CSS
and enter the following code in the box that appears: