I found a Q&D solution to prevent Strings allocating heap memory bei using EXTMEM buffers like that:
Is there any other way, than using this? Or could setBuffer be added to the String class?
Code:
res[n].setBuffer(resbuf[n - stn], 1000); // res[] is in EXTMEM as well as resbuf[][]
res[n] = ename;
//This function does the magic:
void String::setBuffer(char *b,int cap)
{
buffer = b;
capacity = cap;
}