From what I've gathered from Googling, it seems like this would be possible, but I thought I might see if any actual people have an answer. The CTA's own websites were not too helpful. It does seem like I can *only* get a transfer once it's negative, not a whole new fare.
(I wasn't keeping close enough track and it got down to $1.75 on my way home tonight, unfortunately. I don't live near any L/subway stops and my route will be all buses tomorrow so I won't be able to charge it unless I go several blocks out of my way after the first bus to get to a train line.)
Hey, as long as I'm here: Who's excited for the holiday trains? I love when you're just waiting for a train to get somewhere and one of them shows up--surprise Santa train!