I am not sure what your exact intention is by +2.5, I guess what you want is to have an equally distributed vector with a step of 2.5 instead of 5. In general it's good practice to predefine vectorlength, so one approach would be:
x=[-7.5 -2.5 2.5 7.5];
xx = zeros(1,7);
for i=1:3
xx(1,2*i-1) = x(1,i);
xx(1,2*i) = x(1,i) + 2.5;
end
xx(1,end) = x(end)
But much easier would be to define your vector as equal distributed numbers from your minimum to the maximum value with the stepsize of 2.5:
xx = -7.5:2.5:7.5
