Can a landscaper fill in a yard with a very steep down hill slope?

You need to build an engineered retaining wall that should also include a drainage system (ameri-drain). ANy landscaper can do this, however, in order to avoid future litigation, specific details (through engineering process) will dictate specs of the job (footing and post depth, material types, soil types (dicate 'load' on the wall), drainage, erosion, permits for soil transfer.