I actually asked about this a while back and was told it was something that they would be fixing. I assume this means you will be able to do it in the future but it seems that many people are against binding to scroll wheel after the entire little crusaders source movement incident. I have heard the same complaints about lasertag which still has source movement so I am not sure if this is still something they want to add.
You can actually bind it by editing some files. Here is a guide about that. It isn’t really ideal but it is possible.