As an alternative implementation (to doing the addition within one SynthDef) you can work with Patterns. Here are some examples (bottom of the post): Implementing filters with additive synthesis - #4 by dkmayer
Both methods have their pros and cons. From teaching I can say that the Pattern approach is appealing to many as more intuitive. E.g., with SynthDefs you often run into situations where a good handling of arrays is the key to making things more practical. But this can be technically more demanding, e.g., the use of array args, zeropadding etc. These things often cause confusion.