I think you can only do this by manually keeping record of your opponents' nationalities and challenging specific opponents according to this.
Foreign opponents

Click on Play - Online Chess - View Players, and then under Find Player, choose the country and rating you want. If you click on a player's name, it will take you to their profile page, and then you can click on Challenge to Play at the right.
In Live Chess, if you click on Players, and then a player's name, you should see a little flag next to their name, telling you which country they have listed on their profile. If you click on their name in the pop-up dialog, it should take you to their profile page.
I would like to start a series of games (a collection we could say) covering as many countries as possible.
I found a list of foreign players in the Members Section.
Now,
--has Chess.com an especial procedure to challenge any of them?
-- any "ad hoc" procedure for foreign games?
--should I contact them via personal messages?
Excuse my ignorance about browsing in Chess.com, but, really, I am sort of lost.
Thanks for any suggestion.