When it comes to finding synonyms for the phrase "supplementary to", many alternative phrases can be used instead. Examples of phrases that can be used include "complement to", "additive to", "auxiliary to", "accompanying", and "additional". These phrases can be used interchangeably and give the same meaning as "supplementary to". They all express that something is being added on to an existing entity to enhance it or provide something else. The use of synonyms provides variety to written communication, and the right choice of the synonym can help to clarify or enhance the meaning of the entire sentence.