All Custom League commissioners have the ability to edit the rosters or results associated with any team in his or her league. By joining a Custom League, you agree to give the commissioner of that league permission to modify your roster, lineup, or past results at any time without notification or explanation.
As a commissioner, you can edit another team's roster by going to your Commissioner Tools page and choosing the "Edit Rosters" option. Once there, select the team you want to change and the type of transaction from the pull-down boxes. You'll then be presented with options that will allow you to make the desired move(s).
Since you can only control one team at a time, you'll need to return to this page and follow the same procedure each time you want to control another team.