What can I do with dough with the wrong level of hydration?

I've done my fair share of over hydrating, but I'm not too experienced with under hydrating. You could probably do 1 of 2 things: 1. save the dough and use it as a preferment for future challah/brioche batches or 2. I would try cutting the dough into smaller, even pieces, put it in the mixer with the remaining water and let it mix on low until the dough is rehydrated. You should then give it an hour or so at room temperature before shaping. Since its pretty enriched, I don't think you should worry too much about overworking it. You may not end up with optimum volume, but all the work/ingredient won't be completely lost.