The phrase "ancillary to" is commonly used to describe something that is secondary or supportive to something else. If you're looking for synonyms for this phrase, you might consider the words "supplementary," "complementary," "subsidiary," or "auxiliary." Other options could include "supportive," "ancillary," "accessory," "adjunct," or "secondary." All of these words convey the idea of something that is supporting or aiding another, but with slightly different connotations. Some might suggest a higher level of importance or necessity than others. Consider the context in which you'll be using the phrase and choose the synonym that best fits your needs.