Almost all of their flights to/from Beijing are nonstop, so there aren't many opportunities to have stopovers. But, if you are flying something like PEK-SIN via XMN most fares I've seen allow a stopover in Xianmen for a modest surcharge. For domestic itineraries (e.g. via CTU), a stopover breaks the ticket (i.e. you need to purchase PEK-CTU, and CTU-XXX separately).