There probably is easier ways, but here is something I whipped up for you.
Code:
private void button1_Click(object sender, System.EventArgs e)
{
	string[] x;
	string[] y;
	int counter;
	int interiorCounter;

	counter = 0;
	interiorCounter = 0;

	// X is our original array.
	x = new string[5];
	x[0] = "hello";
	x[1] = "@";
	x[2] = "something";
	x[3] = "@";
	x[4] = "@";

	// We need to find out how many elements we are going to be adding.
	for (int i = 0; i <= x.GetUpperBound(0); i++)
	{
		if (x[i] == "@")
			counter++;
	}

	// Now we know how many to add, we need to add the x
	// array amount to the counter variable for the total.
	counter += x.GetUpperBound(0) + 1;

	// Now instantiate our new array.
	y = new string[counter];

	// Start filling the new array.
	for (int i = 0; i <= x.GetUpperBound(0); i++)
	{
		if(x[i] == "@")
		{
			y[interiorCounter] = "@";
			interiorCounter++;
			y[interiorCounter] = "your newly added element";
		}
		else
		{
			y[interiorCounter] = x[i];
		}
                
		interiorCounter++;	
	}

	// Show your newly added stuff....
	for(int i = 0; i <= y.GetUpperBound(0); i++)
	{
		MessageBox.Show(y[i].ToString());
	}
}