You can't easily do it like that - you can do it under some circumstances, but it gets very, very complicated and isn't something I'd recommend a novice trying (or even looking at how to do it).
Instead, why not use an array?
string[] myStrings = new string[] {"test1", "", "test2", "test3", "");
for (int i = 0; i < myStrings.Length; i++)
{
if (myStrings[i] == strings.Empty)
{
myStrings[i] = "00";
}
}
Only I'd do this:
string[] myStrings = new string[] {"test1", "", "test2", "test3", "");
for (int i = 0; i < myStrings.Length; i++)
{
if (string.IsNullOrWhiteSpace(myStrings[i]))
{
myStrings[i] = "00";
}
}