What Kelric says works. Remove the check mark from the inaugural draft setting in the rules menu. You'll also have to turn off "Automatically expand league" and "Hold All-Star Game", and depending on what you want yourself, "disable playoffs" and "schedule spring training".
Then when you've imported the league, you'll need to go into the league structure and remove the four dummy teams (Boston Americans, Philadelphia Athletics, Chicago White Sox, and Detroit Tigers). The other 12 teams will have full rosters. Then move the four in the "AL" to the NL and reset the schedule.
The only problem there is I don't think there is a replica of the real 1897 season schedule for OOTP although if you're not concerned with that you can just keep the one created for you.
Also you can turn off trades and injuries and suspensions if you so wish.
Here's my quick go through to show that it works.
