If your data looks like this:
t = [1 2 3 6 7];
x = 2;
w = 10;
X = t*x + w;
now you can just interpolate to get the missing X values:
ti = 1:7;
Xi = interp1(t, X, ti);
or if you're saying you have this:
t = [1 2 3 6 7];
X = rand(size(t));
then to fill in random values:
Xi(t) = X; %Space out the origianl random value according to t
Xi(setdiff(1:7,t)) = rand() %Find the missing vlaues using setdiff and replace them with new random values
ti = 1:7;