If I understand your question properly you want a + if the number is greater than 0. If that is correct you would need to write a script that says something like:
if(Field1.rawValue > Number(0)) {
this.rawValue = Field1.rawValue + "+"
}
You will need to make your field a text field as it will not accept the + sign. If your field you are getting the information from in my sample Field1 is not numeric then you need to use the Number(0) otherwise it will be treated as a non-numerical character.