Your best option is to speak to either the train staff on the train or ask at the information desk in Frankfurt main station both should be able to speak English German and in this case French on the train. They will normaly let it go no problem but if not a ticket to the airport costs about €6 and with a new ticket you can also use RB. WARNING!!!!! Do not just get on the train and plead ignorance they are not likley to belive you and then you must pay the standard fare same as if you had no ticket and thats expensive!(€40-60).This is a route that is checked daily and more than once a day.
P.s if you buy a ticket get it from the green machines in station they are cheaper than the red machines...