Signing a player on a free isnt allowed in the rules to my knowledge. I tend to approach managers with surplus players with a half value offer - which in the rules is the least amount you can secure the player for

The other choice would be for the manager to free transfer the player. You can then sign him as a free agent (he will appear on the free transfer list) - but you will be competing against everyone else for his signature and he might not pick you

Another option would be to swap him for a player you do not need or similar value

hope that helps