The best way to do this is to make a QuickStart of your league, delete all the leagues and replace them with one WBC league. Adjust the amount of Foriegn players allowed on roster to 0 by team nationality and give each country you want to be represented a league. Than adjust the schedule so each team makes the playoffs and use the regular season as the qualifier rounds. It's a pain to make still doesn't work great but it's the I have been able to come up with. This thread came up a few days ago to show interest in having it added to OOTP18 since that's when the next WBC is.
http://www.ootpdevelopments.com/boar...dded-ootp.html