No, there is not magic bullet soil additive to boost sugar content, but proper levels of soil nutrients, especially potassium, will aid overall flavor. Total sugar content, which varies widely due to growing conditions, is due to amount of sunlight the leaves get, and temperature and low water stress may change acid sugar balance (based on my own observations with flavors changing over the season). The sugar level masks the tartness of the natural acids.
Are you sure the variety is supposed to be sweet? Names don't mean much, for instance Sugar Lump, aka Gardener's Delight, is not a sweet cherry tomato, it is full flavored and sometimes a bit tart/acid.