This may partially answer what you want. Step 1 would be to take the data for "a" and stretch it out as you describe. Here's one way to expand a data vector according to a vector of frequencies:
f = [4 3 0 1 2]; % frequencies
d = [1 2 3 4 5]; % data
c = [0, cumsum(f)]; % cumulative sum of frequencies
x = zeros(1,c(end));
for j=1:length(c)-1
x(c(j)+1:c(j+1)) = d(j); % insert each value as required
end
