How to assign a string-variable to a StackString based on the StackString-library?
StackString doesn't know what a String is.  The append function only accepts a const char *, a const char or an int:

void append(const char* string);
void append(const char character);
void append(int number);


You can get a const char * from a String easily enough with .c_str(), so use that in the append:

myString.append(NormalString.c_str());


There is also no simple assignment (or complete content replacement) with StackString - you have to "clear" the string first.  So to replace the content with what is in NormalString you would:

myString.clear();
myString.append(NormalString.c_str());