Add on codes

Currently setting up Procentive and am wondering how add on codes work in the system.  Example the add on code for interactive complexity (90785) or travel code (H0046).  Have tried to send a few codes through by using the override overlap button in the Time Module on both the service line and the add on line but when I get to the billing module I get an error that the add on codes are not attached to a service line and cannot be billed electronically. Is there a better way to do this?