When I go Documents / New Reading Plan and choose Custom Reading Plan from the floating window, I get the option to add from
- another Reading Plan (works)
- a Passage List
- a Clippings document

The last two options give the opportunity to select an existing document (i.e. a passage list or a clippings file), but don't do anything.
I don't know when we lost this functionality, but I know it worked approximately a year ago in the then-current version of L6. Does everybody sees the same (or is this a Windows 32bit issue)?