You have to go to this location:
My Documents -> NBA Live 07 -> saves -> name of your saved file
Then make a backup of a file players.dbf.
After that, open the file. You have to find a line with a player you want to sign. When you have it, move right until you find the field "TEAM". You have to enter the value that is assigned for your team.
Here you have all of them.
Near the field "TEAM" you can find also the one "ROSTERPOS". The values there go in this order: 0 for starting center, 1 for starting power forward etc. to 14 for IR3.
Make sure that you have at least one free roster spot before doing that. If you add a player as 16th for your team, you will probably end up with a crash.
If something goes wrong, start over from a backup.