The Effects > Make As Waveform action turns a selected group of sounds and turn them into a waveform. You can also create your own composite waveforms by starting from a sample in the composite library, copying it in the piece (using the + button), then editing it (using the square button in the waveforms list).
The pattern "base:granular:additiveBell" is an example of a bell sound generated by additive synthesis. A sample of composite waveforms is available in the "composite" library of sounds. Many are approximation of FM instruments based on Chowning's seminal paper. They are largely perfectible for now.
Note: Because composite waveforms have their own internal dynamic, one may choose to use the "flat" envelope with them, at least to start with and to get a realistic reproduction of the sound.
In the registered version, the composite waveform editor provides some supplementary parameters:
Default pitch. This number represented the estimated pitch at which a waveform is tuned. This value is automatically computed. It needs not correspond to the actual pitch that is heard when playing the waveform, but this is of little relevance, as the pitch value is used relatively to the sound's actual pitch on the main score. One reason to edit this default pitch is to transpose all the sounds that use a given waveform by a certain amount.
Reset pitch: recomputes the default pitch based on the lowest sound shape composing this sound shape.
Recompute: forces recomputation of the total duration over which the waveform is defined. Normally, the recomputation is done automatically when you move sounds composing the waveform around, to encompass all those sounds and only them. Sometimes, however, you will want this recomputation to take place sooner, so that you can keep editing your waveform with an appropriate view.