Oh ok, I understand better ur concern now..!
So u have 2 ways to do that.
Both require u to edit the files "Data\Players.*.Ini" ( * = ATP for Men, and WTA for Women).
1st way = u find some players in the base that play about the same year that the Mexican players u want, and then u change their name & nationality ; this method doesn't need u to start over ur game, but it may give trouble if u remove too much players from a country playing the Davis Cup.
2nd way = u add all the Mexican players at the end of the base ; it'll invalidate ur game, and u'll have to start a new one. Also, u'll have more than 1000 players / year (it's not a problem for the engine, but it means some players will have difficulties to play enough tournaments in the year).
Information about editing the player bases, can be found in the file "Data\HowToMod.html", and in this topic : topic11-953.php