Add a new tag for anonymous structure array

I have an anonymous structure array, say, A.Let us assume the size of
the array is 100. Each structure has tags A, B,C. I want to add a new
tag, D. How to do that in IDL?
I tried:
myStruct = create_struct(A, 'D',0), but it failed.

It seems that the above syntax works only for an individual structure
instead of structure array.
For example,

myStruct = create_struct(A[0],'D',0) and it works.

This is an issue that can be easily solved in matlab. I am not sure
whether IDL provides straightforward method to do that.

Sincerely,
Qi