I don't know how to do it for events already on the timeline, but to automatically add crossfades to multiple events as you add them to the timeline:

1) From the menu bar select Options->Preferences.
2) On the Editing tab, check the box "Automatically overlap multiple selected media when added."
3) Set your desired overlap time in "Cut to overlap conversion."
4) Leave the Alignment setting at the default "Centered on cut."

This trick will only work if you first select the clips you want to overlap using the appropriate combination of Control or Shift plus click, or Control-A to select all. Then drag the entire selection onto the timeline. Note that clips you add one at a time will not be overlapped with adjacent events.