Not sure exactly what you're trying to accomplish, but let me see if I can help.
A couple things to point out first
OOTP now has the ability to import entire historical teams. So you could bring in the 27 Yankees, 84 Tigers, 75 Reds, etc etc and create a league like that.
You can also, I think this is what you want, create an historical league in the desired year. This gives you the teams with the players they had at that time.
During import you'll be given an option for basing ratings off remaining career, peak years, or full (I believe those are the 3 choices). This will help determine what level they are at when imported.
You can use "recalc" to have players follow closely along their real life path/stats. Of course a long term injury can definitely effect that, but it's pretty accurate.
Lahman is built into OOTP. You import historical teams in the legue setup, edit league structure function.
Individual players can be imported on the League FA page.
One VERY important thing to remember - ANYTIME you mess with your league structure, BE SURE to CREATE a NEW Schedule.
I like to emphasize that because so many people forget that step, then when their season starts the schedule is all messed up.
Hope this gets you started at least, feel free to post any further questions here.