As you noticed, "Replace Array Subset" can only replace array elements that exist. If it's not there, there's nothing to replace.
What you'll need to do is make the array larger by using the "Insert into Array" function. Use this function cautiously though. It requires LabVIEW to dynamically reallocate memory while the program is running, and takes time. If you only do the 'Insert' when needed, you should be OK.
Use the "Array Size" function to get the size of the array to be inserted, then you can choose between the ‘Insert’ or ‘Replace’ functions.
Ed