Yes you can do this with a windowless action and associate a window with this action. Create an action in PD manually setting the flag that allows association with window to be true. Design a window for change speciifcally with the attributes you want, like category. Set up a wndow rule to pop this window when the action you created is used. Job done.
I just tried creating a window rule, but it will only let me choose a window from the current process (Change). I need to create a Change Category, so shouldn't I be using a Change Category window?
I suppose I'll have make them create the new change category by hand. I shouldn't really spend any more time on complex stuff like this now. I'm running out of time.
When I saw the word 'add', I thought you meant add an existing category onto a change because the person didn't know what the category was at the time the change was created. I think you actually want to create a new category? If so one of my collegues has done this in the past by dragging the change object onto the change category object and answering yes to the question so a new action can be created. He then had a process action which allowed him to create a category.
In these circumstances, yes you would be using the window for that action, not a windowless change action.
However there were some downsides to this method (which I cannot quite remember as it was a while ago). It think it was to do with the levels of category that could be created.
If time is pressing, I agree - go with a simpler system asking the analyst to confirm they have created the category!