Hello b934cc04acccfc32
Try going back to the edit teams > transfers page. After selecting the club you want to add your players to, press search, then select detailed. Then just click over until you reach men's international, select that, and they should be there, just be aware of the ratings of your created players to help you.
If you created female players, then do the same thing, except since there is no international option there, you will have to narrow down the search based on ratings. Hope this helps.
All the best to you!