Adding a Tail shaft to servo's with no tail shaft

I remember someone posting a process they went through to add a tail
shaft to a DC motor, thus making it possible to put an encoder on it. I
can't find the site in my huge pile of favorites, anyone ever done this
or remember the site I'm thinking of?? I've got my hands on some great
DC motor's but they don't have the tail shaft..

Thanks guy's

Jarrett