You will be able to upgrade your Dynasty Mode staff after every Off-season and before the next Season begins.
The easiest way to get this achievement is to start up a dynasty, pick a team, do all the stuff you like to do, then when you get to the start of the season you press start, then go into Roster I believe it is, and then edit team stats. Now just put your teams stats all to +99 and simulate two Dynasty's. Then you'll have enough points to max out your GM. Oh, and make sure your goal is to win the NBA Championship. And, as an added bonus, you'll get the MVP, All-Star, All NBA, and NBA Finals MVP this way.