Oh dear, oh dear.
How could Mark have added extra eyewitness detail? His gospel is significantly shorter than the other two.
I'm not sure if you mean "how could he have added it, since he wrote after the others" or "how could he have added it, since that would have made it longer (assuming he was third to write)". Although shorter, some sections are longer than the corresponding ones in Mat/Luke.
As we have discussed before, it is highly probable that Mark wrote first and Matthew and Luke copied him whilst adding extra detail (not from eye witnesses though).
Here's another example of why I disagree:
Mat. 15:1-4
Then came to Jesus scribes and Pharisees, which were of Jerusalem, saying, 2
Why do thy disciples transgress the tradition of the elders? for they wash not their hands when they eat bread. 3But he answered and said unto them,
Why do ye also transgress the commandment of God by your tradition? 4
For God commanded, saying, Honour thy father and mother: and, He that curseth father or mother, let him die the death. 5But ye say, Whosoever shall say to his father or his mother, It is a gift, by whatsoever thou mightest be profited by me; 6And honour not his father or his mother, he shall be free. Thus have ye made the commandment of God of none effect by your tradition. 7Ye hypocrites, well did Esaias prophesy of you, saying,
8This people draweth nigh unto me with their mouth, and honoureth me with their lips; but their heart is far from me.
9But in vain they do worship me, teaching for doctrines the commandments of men.
Mark 7:1-13
Then came together unto him the Pharisees, and certain of the scribes, which came from Jerusalem. 2And when they saw some of his disciples eat bread with defiled, that is to say, with unwashen, hands, they found fault. 3For the Pharisees, and all the Jews, except they wash their hands oft, eat not, holding the tradition of the elders. 4And when they come from the market, except they wash, they eat not. And many other things there be, which they have received to hold, as the washing of cups, and pots, brasen vessels, and of tables. 5Then the Pharisees and scribes asked him,
Why walk not thy disciples according to the tradition of the elders, but eat bread with unwashen hands?
6He answered and said unto them, Well hath Esaias prophesied of you hypocrites, as it is written, This people honoureth me with their lips, but their heart is far from me.
7Howbeit in vain do they worship me, teaching for doctrines the commandments of men.
8
For laying aside the commandment of God, ye hold the tradition of men, as the washing of pots and cups: and many other such like things ye do.
9And he said unto them, Full well ye reject the commandment of God, that ye may keep your own tradition. 10
For Moses said, Honour thy father and thy mother; and, Whoso curseth father or mother, let him die the death: 11But ye say, If a man shall say to his father or mother, It is Corban, that is to say, a gift, by whatsoever thou mightest be profited by me; he shall be free. 12And ye suffer him no more to do ought for his father or his mother; 13Making the word of God of none effect through your tradition, which ye have delivered: and many such like things do ye.
Notice the blue highlighted sections, and how the thought process is smooth in Matthew but interrupted in Mark. Which is secondary?
As an exercise, why don't you find some examples of alleged eye witness testimony in the gospels and explain to us how you know they are eye witness testimony.
Because if I do you will just say, "well, fiction writers can make up detail too"!