The expansion teams are just dice with different results on but implemented in a clever way and I like that they don't use proper NFL teams as it wouldn't really seem right. If you wanted to though, I suppose you could do it yourself but I don't believe there would be a good balance. See my review on what I think of the expansions and why.A, there would be serious licensing issues and costs for getting the names and logos of the actual NFL teams. I don't think that's feasible for a small game.
B, teams change from year to year, thanks to the draft, player trades, coaching changes, etc. Having to tweak the intricacies of each team's strengths and weaknesses from year to year, especially using an abstract system such as this that would inevitably be disputed by football fans, would make maintaining the game's "realism" also an unattainable goal.
C, let's face it, the current NFL teams are not balanced, despite the league's best attempts at parity. So some teams just wouldn't get played except by people who like to play the underdog.