How to append string

I've been looking through srting functions for an hour and now know how to do everything besides what I need to do.

I have an arrray of strings. I want to append another string to a string in one element of the array. I'd just do it this way (or
something like it) but I can't get it to work:

array[3] = array[3] + "more string";

It doesn't seem to work like C, java or javascript languages.

Thanks,
Dale