CIC SINC3 Filter with unsigned Integrators
Addition and subtraction in 2's complement or unsigned is the same. The only difference is for overflows.

Assuming 16-bit arithmetic, overflows occur in unsigned addition when you go over 0xFFFF (65535). For example, if you add 1 to 0xFFFF you will wrap around to 0.

In 2's complement, the overflow will technically occur when you go over 0x7FFF (32767). If you add 1 to 0x7FFF you will wrap around to 0x8000 (-32768). 

Notice however that 0xFFFF + 0x0001 = 0x0000 and 0x7FFF + 0x001 = 0x8000 whether you're in 2's complement or unsigned. 

Therefore, it does not matter whether it is implemented in 2's complement or unsigned, the behaviour will be the same.