Yes, CULT+KING is great for extensions because non-synthetic extensions need and thrive with the botanical oils in products like C+K. Unlike natural hair, extensions do not get sebum from your scalp, because they are not attached to your scalp. Lack of sebum makes hair more fragile and dry. Oils give hair a temporary protective layer similar to natural sebum, that works great on both natural hair and on hair extensions, including making extensions appear more natural.
BUT it is very important to differentiate between different kinds of extensions, and to apply C+K products properly for each type of extension.
For Beaded, Sew-in and Keratin-bonded extensions: you can use all C+K products with no issues as long as the hair is not synthetic.
For any extensions that use adhesives, like Tape-Ins: CULT+KING products can be used, but carefully so as not to apply C+K directly to the adhesive area. It's important to avoid applying products containing essential oils to these areas as the oils may weaken the adhesive. BTW, remember that all CULT+KING products contain essential oils with the exception of SUPERWATER.