I learned something new, the SHIFT key overrides the default radius. So when prompted to pick lines to fillet hold the SHIFT key down to fillet with a zero radius. This will keep your radius stored & will not over write it.
I used to use FF to execute a lisp routine that would fillet with a radius of zero. You can create a toolbar, menu or tool palette object to do this but what about the keyboard. Here's a tip.
In the CUI drag the command fillet up to the keyboard shortcuts part of the CUI tree.
Once added you can adjust the macro to set the fillet radius to zero & specify a keyboard shortcut to execute the command, I use SHIFT + F to execute it.