I have no experience with Max Msp, but this seems to be the answer: https://cycling74.com/forums/relative-controller-assignement
In relative mode, you just sum (accumulate) all values you receive. There's a catch, though: all values are encoded as 7-bit two's complement. So if a value is greater than or equal to 64 (0x40), you have to subtract 128 (0x80).
0 → 0
1 → +1
2 → +2
...
63 → +63
64 → -64
...
126 → -2
127 → -1