Look, a WR can play CB with help [Troy Brown had injured Ty Law to help him] and talented CBs can play WR [Prime Time.]
HBs can come out of the backfield and play WR, and so can TEs. TEs can also play FB. Tackles and Guards can sometimes play TE. Some O-Linemen in the NFL used to be TEs in High School and/or College. On that note, I used to love moving Joe Stayley [49ers LT] to backup TE in Last gen Madden 08. He would be a 6th lineman in big formations. He also was able to catch some open passes, so he was a valuable role player for my team in that game.
OLBs can sometimes be 4-3 DEs and vise versa. Some Strong Safeties can come down and become Line Backers, or vise versa. For example, Troy Polamalu, Bob Sanders, and Rodney Harrison often play[ed] like 4th or 5th linebackers. Corners can become Free Safeties [Ronnie Lott and Rod Woodson.]
Does that help answer your question?
From a simulation standpoint, just look at the player's individual ratings and see if they have the skill set. Also look at the physical build of the player and see if they look like they could pull double duty. FOr example, you may find a rookie in a draft clas that looks like he could be a hybrid player. Maybe edit/raise his ratings that apply to the new position a bit to represent that and have fun. Just don't go over board
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P.S. Did you know Teddy Bruschi came into the NFL as a DE? He was later moved to OLB and then MLB.