An example would be useful. I will try fo figure out one.

Son fusil enrayé, il sortit son arme secondaire.

Perhaps deuxième.

Son fusil enrayé, il sortit sa deuxième arme.

I don’t feel this deuxième weapon is accessory.

Perhaps complémentaire, but the sentence won’t be natural. It will more likely be:

Son arme principale enrayée, il sortit son arme secondaire.

This arme secondaire is not accessory, here it’s the correct word in French.

Others candidates: alternatif (somewhat critized in formal French because une alternative is not another option, but both options as a whole), adjoint, associé, d’appui, d’appoint..

I think the word you are looking for is “dérivé“.

Example:
“Le kérosène est dérivé du pétrole brut”

I don’t think there is a word that fits better than “composé”, so I’ll just stick with that.
Thank you all for your suggestions though.