Here's a Custom Team Tutorial..The best tutorial i've seen before was from dreamspeaker from nbalive.org...but since their forum is down i'll post one here.
Let's say you want to create the greatest NBA team ever...The 1987 Lakers

open up the Teams dbf in one of your saved rosters.

Copy the whole line with the lakers...team number 12
create a new entry and paste the line you copied.
Change the following values to these numbers
Cityname ('87 Los Angeles)
Teamnum 51 (since the last team in the dbf file is team number 50)
Teamcode 13
Seed 0
Conference 2
Division 6

This will make your team a custom team which you can only play during exibition mode.
Add players to your team
Let's add Magic Johnson to our team
open the players.dbf
Copy the line with Magic Johnson's player id (playerid 740)

create a new entry and paste the line you copied

Open the appearance.dbf and search for appearid 740 and copy it
add a new record and paste magic's appearid .
Change the appearid to 2383
go back to players.dbf and change his playerid to the same number...2383
Keep creating players until you have at least 8 or the game will crash.
Add the coach
Open the Coaches.dbf
Copy the line with Pat Riley
add a new record then paste his info
Do not change his coachid...the coachid corresponds with his speech files.
Go to the line that says team..and change it to our team number..which is 51

Add Jerseys
Open up the teamgear.dbf and find the jersey files.
For this tutorial i'm just going to use the default home /away jerseys instead of the retro 87 ones.

copy the home and away jerseys and add them to the dbf
Change the id numbers 381 for Home and 382 for away
under jerseytype make sure it's set a 0 for home and 1 for away

That's pretty much it for creating a custom team