You can do it if you have the players.
For instance take WE United, change team name, team logo (you can choose from all national flags), team kit etc. Then register the players, and remove the players you don't want. Since I think you find 23 Trinidad & Tobago players in the game, you'll have to create them. It's easiest to overwrite players then creating new ones. For instance you can overwrite some WE United player, change name, appearance, stats etc. into the Trinidad player.
When you want to play a 'World Cup', you have to choose Konami Cup and set up the tournament format because when you choose international cup you will not be able to select your new countries, since the game will regard them as club sides.
I did not make any new countries except Classic Spain, so I can play a 8 team tournament with the other seven classic teams.
Good luck