Yes, they stack on top of each other. Some actions though can only happen in finite manner (like 1 official visit per player for the recruiting period or you can only have 25 total home visits each week).

Portal Players (Transfer) are only available during the offseason recruiting period. Generally they are players who decided to transfer from a team due to lack of playing time, coaching changes, or what not. You recruit them the same as you would a high school player with the main difference being you can see all of their attributes and skills.