They may join your club, but they must have a work permit before they can play. If you have a Belgian club as an affiliated club, you can loan that player out to them until they do gain a work permit.
It should take roughly three years. Once they have, they are then eligible to play for you. We say Belgium because it's one of, if not the best country in getting work permits within the EU quickly.
Belgium and Bulgaria are the best as they have 3 years continously req , Always load both leagues so your prospects don't get simulated , if you don't have em loaded they will not develop well