Arduino: String to char Array
Sounds like the following string.toCharArray(buf, len) is what you want. as exampled

String stringOne = "123456789";
char charBuf[50];
stringOne.toCharArray(charBuf, 50) ;
Serial.println(stringOne);
for (int i = 0; i &lt; 9; i++) {
  Serial.print(" 0x");
  Serial.print(charBuf[i], HEX);
}
Serial.println();


yields:

123456789
0x31 0x32 0x33 0x34 0x35 0x36 0x37 0x38 0x39


as each contents of charBuf is an uint8_t, regardless of how it is printed. 

Unless you are asking for each element of the array to be another string of the ASCII representation of the HEX. or are you asking to have a char array of "0x31,0x32,0x33,0x34,0x35,0x36,0x37,0x38,0x39"