To make someone a floating spare:

Go to Menu > Manage > Player:

1. Select the player

2. Click on the division on the right of the page.

3. Click on the Pencil.

4. Check the Floating Spare box. 

5. Save.

*If the player was already put on a team roster. You will want to remove him from the team.

How do I remove a player from a team roster?

What is a floating spare?