Adding a manual switch for coolant fan, is there a way I could wire in another relay for this to work without throwing a code?

The fan should already turn on when you turn on the A/C. However, that fan doesn't run constantly like they used to, instead they monitor the HVAC system using the PCM and its sensors as well as the pressures and A/C operation to cycle the fan on and off as it is determined to be needed. That is done to improve fuel economy as the pans do draw quite a bit of power. As for adding a switch, you could, but it won't change the operation of the A/C and in reality, may make it worse in modern designs. Also, to answer the question, yes it would set a code, most likely a thermostat cover as the PCM would know it isn't cycling the fan, but the coolant temperature is going down. Under normal conditions the low-speed relay will be the one that turns on first and then cycles off.
If the A/C doesn't seem to operate very well, you might need to service it.