[Brad K]

1. Obtain a players vs L only stats.
2. Enter them into the stats section of the editor of a test player
3. Select "calculate ratings from these stats" or whatever close to that it is called.
4. The resulting "combined" ratings are the player's ratings vs L.
5. Record the ratings, BABIP, Avoid Ks, etc.

Repeat this for vs R

You now have ratings for the player for both vs L and vs R. Now you can create your actual player with these ratings. Put these ratings in the vs L and vs R columns and you're done.

The resultant stats in OOTP will likely not match real life stats because the stats shown on OOTP are weighted 25% vs L and 75% vs right. It is highly unlikely your player's actual matches this weighting. But it's irrelevant. The game runs on the split ratings not on the combined ratings or resultant stats.

Before the game offered real splits the choices were no splits or random splits. Random splits were created around the players overall rating based on some average difference between vs L and vs R with possibly some randomization thrown in. Dissatisfied with that I created a spreadsheet so I could convert a player's overall ratings for the season to L/R splits which applied his career splits (to get a good sample size) to his overall ratings for that season.