if u only want to add those Japanese players into NBA without adding japan teams, u can first load the NLSC roster and create some players, save the roster at a new file and quit game, go back to windows and backup your rosters.
Then use DBF viewer to open the two rosters at the same time, copy the japanese player to the NLSC file and change the playerid to make sure than no overlapping with current players. Need to copy both Players.dbf and Appearance.dbf.
Finally copy the CF files of those players into SGSM, if the file overlap the current NBA player, change the name of the file as well as change PLAYERPKG in Players.dbf.
If u wanna add the Japan high school team inside, thats another story