B(3) = A;

%defines input stract
a.f1 = 2; a.f2 = 3; a.f3 = 'str';
temp.f2=5; temp.f3 = 7; temp.f4 =1;
b(1) = temp; b(2) = temp; b(3) = temp;

%iterates over the fields of a and assigns their values into b
fields = fieldnames(a);
for ii = 1:numel(fields)
  b(3).(fields{ii}) = a.(fields{ii});
end

a =
f1: 2
f2: 3
f3: 'str'

b(3) =
f2: 5
f3: 7
f4: 1

b(3)=
f2: 3
f3: 'str'
f4: 1
f1: 2