We probably just need to change the titles and descriptions. The existing categories were not meant to be for the camera manufacturer branded lenses only (i.e., any lens that can be used on a Nikon dSLR is fine in the Nikon lenses forum).
We don't want to make many changes to anything in the existing software setup right this minute, as we're working on a project to enhance the forums right now (and we are looking at ways to do nested forums for more granularity, which may be one approach to third party lenses). Another concern is that when you start getting too many forums, posts are more spread out when someone is looking for something.
For example, someone is looking for a bright zoom for sports would have to look in multiple forums to find out what may be suitable, versus a single lens forum for any lens that can be used on a specific camera mount). The amount of posts in a given forum is also a consideration, as the traffic may not warrant separate categories. For example, we haven't even added a separate lens forum for KM/Sony dSLR models yet because there isn't really enough traffic yet to justify it (and more forums increases page load times).