If its anything like the PS3 I believe you can download the update to a memory stick then add it but I'm not exactly 100% sure
I haven't gotten any recent roster updates on the PS3 either. Mike Williams is still a raider on mine.
you will need live to get the roster updates on 360. just start a quick game against anyone and it will download.
When you find the play you want.. Press and hold A on that play then go through the playbook while still holding the A button and when you get to another random play or whatever just release and the original play you held the button on will be your play