Hello Atul Sharma,
As far as your query is concerned, it is a very valid query. Here i would like to mention that usually power combiner is used to combine power levels of the signals may be one two or n number of signals. It also depends on the system applications where and what for you are using it. Generally it is for combining of two or more signals leaving out power perspective. And as far as Add module is concerned, it is used to add another wavelength to the existing wavelength. In optisystem particularly , it has only two inputs. But generally speaking , it can very well serve you as a combiner ( not sure about power levels but it will combine your signal to existing signals ). I hope it is helpful.
Regards