Check out the two threads below, started by Tiger Fan some years ago. They describe how you can add - for example - the Pacific Coast League much earlier than its OOTP beginning and more like when it actually started.
https://forums.ootpdevelopments.com/...d.php?t=286497
https://forums.ootpdevelopments.com/...d.php?t=276256
In the end, what you want may be possible, but it'll require significant manual work.