If you have Clemens somewhere in your roster, you can do it by renaming another player, some created player by the game, and change the player's name to Roger Clemens using his Audio/Photo and Face numbers, and also edit your "Roger Clemens with what you want.
If you are in a Dynasty and using a roster without Roger Clemens, you probably can do it as long as his Audio/Photo and Face numbers are in the files. In other words, if you replaced the original roster and Clemens is not there but his Audio/Photo are still in the files you can do it. If not, I don't know how to add a player, but there may be a way. Hmm...perhaps creating Roger Clemens and find some face and portrait files in the download section, and also audio if you don't have it now.