[The Roamin' Umpire]

Remember that there is one further restriction, NF 9-6-4c: It is illegal participation... to use a player, replaced player, or substitute in a substitution or pretended substitution to deceive opponents at or immediately before the snap or free kick.

Yes, they may insert their 11th man as a receiver after the other 10 players are already set, provided he meets the other restrictions. However, if the offense appears to be trying to sneak the player in (especially if this happens more than once), we should shut the play down and toss the flag for IP.

A new NCAA rule this year is even more explicit about this, requiring time for the defense to make counter-substitutions and including a possible delay flag or a USC on the offensive head coach.