Here's one idea. Try this out on your test server to see if this suits:
In Object Designer drag the Incident Management - Category object onto the Knowledge management - Article object and select No when asked.
Then modify your article window to add the new category field onto it.
Modify your articles to specify which incident category you want them to be associated with.
Go into Query and Report designer and create a new System Filter.
Select to base it on Knowledge Management - Article but filter by Incident Management - Incident.
Drag on the attributes from the article that you want to be displayed to the user.
On the criteria select your new category attribute on the article and the the condition "Is Equal to Category" and specify a runtime value of Category (from your Incident object).
Pop this filter on the window in question and this should populate ok when the incident category is selected.
Small disclaimer: I haven't tested this in portal.