Can you use Serial Port as a variable?

Majenko's answer is the right answer for your question. But to answer the title of the question, if you ever need to use different Arduino outputs and inputs as variable, most of them have common type Stream or Print. So it would be:

Stream& SensorSerial = Serial;

or

Stream* SensorSerial; 
SensorSerial = &Serial;

The Arduino Stream classes hierachy:

Yes. The simplest way is with a preprocessor macro. Macros get replaced verbatim before compilation happens, so you can do something like:

#define MY_SERIAL Serial

void setup() {
    MY_SERIAL.begin(115200);
}

void loop() {
    MY_SERIAL.println(millis());
    delay(1000);
}