Priority is a calculated field. So based on the priority from the email, you should set the Impact and Urgency on the incident. You can do this using the following format. Add this to your existing inbound action script:

You can add as many else if statements as you have priorities

if (email.subject != undefined) {
    if (email.subject.indexOf('priority - 2') != -1){
        //OOTB impact 1 and urgency 2 = priority 2
        current.impact = 1;
        current.urgency = 2;
    }
    else if (email.subject.indexOf('priority - 3') != -1){
        //OOTB impact 1 and urgency 3 = priority 3
        current.impact = 1;
        current.urgency = 3;
    }
    else if(){
        
    }
}