Install xdotool and xbindkeys:

sudo apt-get install xdotool xbindkeys

Create a binding in $HOME/.xbindkeysrc for e.g. Alt+:

"xdotool click --clearmodifiers 3"

  alt + b:1

And then start xbindkeys (typically, this would go into your xinit file so that it is started everytime you log in.)

Now the problem here is that although it should work with the --clearmodifiers option, something (Unity? Compiz? The global menu?) is preventing the alt key from getting released. An ugly workaround (until someone gives you a better answer) would be to add a sleep 1 and release the alt key yourself:

"sleep 1; xdotool click --clearmodifiers 3"

   alt + b:1

So: if you press Alt and click the left mousebutton, then release Alt within 1 second, you will get a right mouseclick (although with 1 second waiting time).