I've recently added generic convolution filter to the NVidia Textures Tools version I'm using at work. (This is based upon unreleased version that I grabbed from NVTT Google Code page. I wanted to contribute it back to the code base or share with the public, so I asked for the permission from the work, which I'm still waiting for.
At the end it's not hard. If you just dig around the source code, you will find a way to implement it in less than an hour. Or, just wait until I get the OK sign from my work :)