I'm pretty sure it's not. I've never heard of anyone having done so successfully.
The BB will only recognize properly formatted fonts. In other words, fonts that are packaged with your OS. You can add additional BB fonts (5.0 fonts for instance) by placing the .cod file in the java folder of your current OS and then running Loader.exe. But you would be limited to fonts that are already available from RIM.
Both are correct except, if you can find a theme maker that is willing to use these. Then these fonts would be on your BB, otherwise no way Jose.