He can. you just have to offer him a contract and change the contract type to player/coach, player/gk coach or whatever you prefer, if that's what you mean. (I find it rare that they actually accept the contract by the way)
If it says they may want to be a coach when they retire then you have a private chat with them and say they would make a great coach. If they want to keep playing football then thats fine but if they are a bit older then they might say "I could start now if you like?" If they say that then their coaching stats appear and you can offer them a player/coach role.