technically I believe No you cannot add players to the 3 rows that are defined as the scrum added to this is the fact that a contested scrum cannot take place unless the front row contains the trained props and hooker.
A back row is the last row of seats in a theatre, or the row of players in a rugby scrum, consisting of two flankers and a number eight.
Prop forward Scrum half
The is a front row player in the scrum - The hooker whose role is to throw in to line outs and in scrums attempt to"hook" the ball back and win possession
Front row: pawns Back row: rook, knight, bishop, queen, king, bishop, knight, and rook
six, the front row has three positions two at the line closest to the net and one at the net aka the setter. in the back row there are also three, two about three feet behind front row and one pushed back to the back line about a foot ahead of it
There are no actual figures readily available for this however, nearly every forward in both codes has suffered an injury at some point when scrummaging. This can be anything from bruising to sadly near fatal injuries. The front row are particularly susceptible to injury which is why their positions cannot have any other position taking the role on they must be from row specialist or the scum becomes uncontested
Since last week there must be one for each of the front row positions. 1,2 and 3. Any other five positions may be covered.
there are 15: tight head prop, loose head prop, hooker, second row, second row, number 8, open side flanker, blind side flanker, scrum half, fly half, centre, centre, wing, wing, full back.
I assume you mean in the middle of the front row as there are no other positions associated with this. The player is a hooker
They usually try and keep the same 14 players on the pitch at first but when there is a scrum they take a player off the pitch and put on their replacement prop
1 - the "Front Row" consisting tighthead prop; hooker; Loosehead prop [3]2 - the "middle row" consisting openside flanker; lock; lock; blindside flanker [4]3 - the "back row" consisting of the "Number 8" forward [1]so a total of [8] in a scrum.