Currently, as far as I can tell, the only way to define Ground Services at an airport is to add a Tower and Ground Services function, which requires a Tower.
Many uncontrolled airports do have a local FBO with their own frequency, and the FBO will typically be full service, including Pushback, and ground power, fuel, etc..
Please add an FBO frequency to the <com> section that then allows Ground Services to be created, including Pushback and Fuel and all the others.