Hello,
It is possible create a String that concat bytes indeed ascii characters ?
For example,
String x = "";
byte b = 12;
x.concat(b);
for(int i=0;i<x.length();i++)
Serial.println(x*,DEC);*
So, the output is 49 50 
the method concat() is perfect to my propose, any suggestions ? THANKS.
No, you don't want to use the String class for that. If you want to convert a number into ascii digits use itoa.
char x[4];
byte b = 12;
itoa(b, x, DEC);
//printing chars will treat them as ascii and print '1' and '2'.
// to get the requested output we will cast to int so they are
// printed as numbers 49 and 50.
for(int i=0; i < strlen(x); i++){
Serial.print((int)x[i];
Serial.print(' '); // print a space to separate values.
}