Though SSCSSS generates samples by adding sine waves, usually additive synthesis means being able to control the frequency and amplitude of the sine waves over time. Since SSCSSS generates single cycle samples, it does not provide a way of modifying anything over time. But ...
It can help.
Setting up the frequencies for harmonics can be difficult using only the EX. SSCSSS can create a set of waves corresponding to a set of harmonics.
By creating several waves with the of the same "base" frequency but only one harmonic each for harmonics 1, 2 3, to however many you want to work with (up to 4 in voice mode, 64 in performance) the initial frequency can be set to harmonics of the "base" frequency. Then, modified by using EX5 capabilities such as EG's, LFO's, or MIDI controllers.
In song mode, all parts set to the same MIDI channel will play together. The same thing can be done in performance mode if local control is disabled and MIDI out enabled and looped back. See Derek Cooke's tutorial EX_Layer-Split from sample library (http://www.ampfea.org/sln/). This allows up to 64 sine wave elements.
Note, the EX will have trouble starting that many elements simultaneously and 64 elements are a lot of elements to configure.
Another note -- with 64 harmonics, aliasing will start a little above middle c and be obvious after the c above middle c. Amplitude scaling may be desired here.
The doc/examples directory has two files, addwave.in and addwave.s1y, that can be used as a template for setting up 64 waves in a performance like this. Since the addwave.in file uses the low level method, there are warnings on aliasing, but no error and the s1m file is generated.