I recommend to get the player version, you won't regret it
Though, I can't convert a .iff file for you. Let me explain
When it comes to created players, there are values in the game that shape the face (this is the sliders that you adjust when you create a player, the slider you move gives it a numerical value). Though on REDitor there is a column you can change that overrides the NBA sliders and you can assign a PNG file to the player. So if you create a player in the game, they TECHNICALLY don't have a PNGfile because all the numerical values are in the game already to shape the face. When you select the column and assign it a face, THAT is when you can put a value in the CF ID column (such as the number in the PNG .iff file)
So there is an on and off switch when it comes to faces. ON means you can assign it a face and off means NBA will use the numbers stored and make one for you.