You have to open players.dbf file from your save directory. Then find the player you want to be in your team and in the column 'TEAM' replace the old number with new, which represent your team. Also pay attention to the column 'ROSTERPOS' which mean the player position in your roster.
To find out more, you can read the
Live 06 players.dbf editing guide, which is almost the same as the Live 07.