Insert a char in string

[Alexandre via Digitalmars-d-learn]

I have a string X and I need to insert a char in that string...
auto X = "100000000000000";

And I need to inser a ',' in position 3 of this string..., I try 
to use the array.insertInPlace, but, not work...

I try this:
auto X = "100000000000000";
auto N = X.insertInPlace(1,'0');