It's not possible without dbf editing. Even then, the results aren't perfect as you can only access the PDA for one team (usually the one that comes first in alphabetical order) and the game often crashes.
I assume they use Season Mode to sim games since that allows you to select multiple teams. Otherwise they could edit the dbf to take control of multiple teams in Dynasty Mode without worrying about the negative side effects such as not being able to read the PDA for each team since that wouldn't be necessary to run a sim league.