It does support concatenation, but not that way, as i know.
For that you use the Gs function: str_cat( string to pointer, string from pointer)
the string from pointer can be like this.. str_cat(str1,"second text added");

to retrieve individual chars, you can use str_trunc and clip, manipulations, though i think there might be a high chance ( i didnt explore yet) about char[char-number] options.