How to make manual upsampling?

I want to add a zeros vector each "w" elements of a given vector.

fs=20;

N=50;
T=1/fs;
n=0:1:N-1;
x = exp(-0.5*n*T).*sin(2*pi*n*T);
A=zeros(1,M);
for i = length(x)
  y()=A %Here is where I don't know that to do
  end
stem(y)

So As an example if M=2, A=[0 0 0] and if x=[1 2 3] with w=1.

y=[1 0 0 0 2 0 0 0 3 0 0 0];