http://iscoresports.com/

How do you add a player who is not currently on your roster, after you've already started scoring a game??

Go to Misc | Show Starting Lineup. You can edit the starting lineup from there and add players to the roster. Keep in mind there are some limitations you'll run into based on if the team has already batted through their lineup before you make an addition, but if you are adding a non-batting player (Batting = No), then it shouldn't be a problem.