Go into Season mode, go to Commands (I believe) and select Reg. New Player. Then scroll over to your Success mode player area and pick him, then hit Ok. Now you have to pick him up through the F/A Acquisition.
It'll unlock as long as the trophies are collected. Obviously, you won't be able to purchase it until you get the points, but it will be there in the event that you do get the points.
To expand on Darkshade: Actually, Reg. New Player is under the <Management> section. Choose that option, and choose "Register" (Pink color, under "OK") select the player you want to add, and put him into the space up top. Then, just sign in as a free agent, just like you would if you wanted Micah Owings.
Another way to use updates rosters would involve you losing almost all of your owner points in Season Mode. You would Trade all the players you don't want, for the real ones, and you would probably need to add in a bunch of owner points to make the trades work. Or you could just hope to get really lucky at the dream draft!