Yes there is. I can't remember. I haven't really played around in the MLS at all in this years game. But what Botty is trying to say is when you offer a player to clubs and you get offer's from other teams. When it is for MLS club just counter the offer. Then you will be able to add international slot. But be careful cause there is another option for how long. It is set to default for just one year. There is an offer forever, but I would advise not to add in picks from the draft if you are trying to get an INT slot.
EDIT: If that isn't the correct way it is very similar to it. I am trying to go off memory when I was looking for another way to get it for someone else, but that was the only way was like that. Hope that helps.