Sorry, but you can't do that in the current version. I know that Raphael was going to write up some thoughts on how to support this in 3.0 but I don't know if it's gone any farther. [originally posted on jforum.net by monroe]
FWIW, since it seems like you have a fairly consistent and "parsable" naming convention, you might be able to "fake" it using some FreeMarker code. E.g., In the forum index page, you can modify the template scripts to add in the HTML to show the "19xx" psuedo category prior to the actual forums.
In the admin, the grouping is still by the top level category, but in the index looks like sub categories.
This one time, at bandcamp, I had relations with a tiny ad.