Well, let me ask you first, how do you feel towards your ex right now? If you think you are ready to accept nothing but friendship without fooling yourself, then yes, I don't see why not. I know some people who were ex-lovers and are now excellent friends. But if you know in your heart that you still have feelings for your ex, it is stupid to think that you can be friends and worse, you will only hurt yourself. If you are mad at your ex, friendship is totally out of the picture. My suggestion is to move on first, and when you do that, then you will be ready to stay in the friend zone.