http://iscoresports.com/

I was scoring the Cardinals / Reds game and was not aware when the game started that Corky Miller had been called up from Triple-A, so I did not have him on the roster.

When he was subbed in late in the game, I wasn't able to find a way to gracefully handle this. I know that post-game, I can go to the pitch by pitch and make edits - but is there a way to add someone to the roster and line-up once a game has started?

Just go to Misc->Show Starting Lineup, and you can add additional players to the Lineup for the game. Just set him to Batting=NO when you add him, and he will be available as a sub. After adding him, you can do a substitution as you would at any other time.