Custom 'Smart Lists' (e.g. Unplanned)?

Is it possible to create your own 'smart lists'?  I find them very useful and would like to create some additional ones.  For example, in addition to the built-in 'Planned' list, I would like an 'Unplanned' list that would include tasks that weren't on today's to-do list but needed immediate attention. This would allow me to see how much of my day is getting disrupted by things I wasn't expecting to have to work on.