This means you cannot use frags yourself but it doesn't matter if you get fragged by someone. Although this is quite easy to pull off on most of the races in the Career, you will find yourself fragging a lot for the city targets anyway therefore you can use the previously mentioned track.
Go to World Cities in Multiplayer and search for the creator "igor5618" click on the "Easy Prey" city and choose Short. This track is roughly 30 seconds. You do not have to load the event over and over again, you can select the "retry" option on the post race screen and repeat.