There is a limited amount of space available for the player names in the gamesave file. By default all the players in the game (teams and free agents) do not share names. But the draft class player names are basically 'pointers' to already existing player names.
For the draft class I would suggest the following:
1. Decide which players you would like to add.
2. From those players look to see which names are not in the gamesave you are targeting.
3. Edit the worst Free agents you can find to distribute those names.
4. Apply your draft class.
Here's an example from the
TSBTool thread: