Full adder (FA) base 8(Octal)
Doing an octal adder is no different than any binary based adder, it's all just in how you group the digits.

Hex, with 4-bit groups:

4(base 16)    0100
6(base 16)    0110
              ----
A(base 16)    1010


Octal, with 3-bit groups:

4(base 8)    000 100
6(base 8)    000 110
             -------
12(base 8)   001 010


Exact same bit patterns (1010 vs 001 010).  Circuitry is exactly the same, just grouped differently.  You should be able to finish it from here (I'm not going to show you a circuit for a FA).