According to William Whitaker's Words it does:
adjunctum, adjuncti N N [XXXCO] quality, characteristic, essential feature/attribute; collateral circumstance;
However, I'm wondering if it might be an error because it seems contradictory to speak of something as both collateral and essential.
According to Lewis and Short, the various definitions of adjungo involve the idea of something joined or added to something else, which would be inconsistent with it being essential.
Finally, adjuncti in the following sentence was translated with the English word adjunct, along with a footnote defining it as "Something added to another thing but not an essential part of it":
In Scripturis vel promissionibus, enunciationes continent et exhibent objectum fidei, vocanturque objectum fidei per metonymiam adjuncti.
After consulting various Latin dictionaries, Whitakers was the only one I found that explicitly mentioned this question concerning it being essential.