In LM, a sub must be subbing to allow entry of a score card, but subs can play across divisions.
So to solve this problem...create a 2nd division for your league.

This divisions is only being created so that subs can "sub" in this divisions and score cards can be posted.

When a subs plays (that is really not subbing for anyone)...let him sub in division 2...everything else works the same.

To create a 2nd division (Division 2)....

1. Select "Create New League" on the LEAGUE menu.

2. Highlight Existing League and click the [New Division] button.

For more information, please refer to Creating A League.