You don't store data directly on the NKIssue object itself. NKIssue has a contentURL property that is unique for each issue, and points to a writable path that you can use to store your content. How you store it (plist, sqlite db, flat files) is up to you.
You could subclass or add a category on NKIssue to make accessing your app-specific properties easier, but it's still up to you to determine how they are stored on disk.