It is possible to store custom Stateflow charts as a whole in libraries for use in multiple models. As far as Stateflow objects go, they cannot be stored as a stand-alones in libraries, but they can be placed inside charts and then stored. If Stateflow objects need to be used across multiple models, it is worthwhile to look into Atomic subcharts.
Documentation for Atomic subcharts - <http://www.mathworks.com/help/stateflow/ug/what-is-an-atomic-subchart.html?searchHighlight=atomic%20subchart>
