The thing is, they pick players from teams 'un-playable' in the game, such as non-MLS teams, eg. Atlanta Si;lverbacks etc. And so you would not pick players from other teams, unless those teams were in a lower league.
Also, the US league is very delicate, and even the smallest mistake which might go un-noticed on another league, would crash the game in the US one, and the Drafting might be hardcoded into the game, which is practically impossible to change.
If you were lucky you might be able to make it work yourself, but I doubt it.
Hope this helps.
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