A player can be added to the practice squad if he has less then 3 acrued seasons in the NFL. You can activate a PS player to the 53 man roster but someone has to be cut or put on IR to make room.
That same player can be cut again but must go thru the waiver process. If no claim is put in for the player during the waiver process and the player clears waivers, he is essentially a free agent now. Free to sign with any team or any teams practice squad, not just ours.