You can also give the heat generation rate the center of the sphere within the fluent itself. If you make any point inside a sphere and name it..so that in fluent you can easily put it as heat generation point and provide the heat generation rate.

I have done such type of case to simulate the microwave oven, where i provide the heat generation rate to the entire heating element of a oven.

It is suggested that you define a certain smaller sphere in diameter inside your big sphere by "Adapt" in fluent, and assign source in this domain. It is not recommended to use single point heat generation source, which is easy to produce temperature abnormal points.