Like any other i-adjective, they can be conjugated but I read a few example sentences of the i-adjective "無い" and I can't find any sentences where it was conjugated.
You can check out this pretty thorough listing of ない conjugations here.
According to the link:
- ない is the Terminal form as well as the Attributive form.
- なく is the Continuative form as well as the Adverbial construction.
- なかろ is the Imperfective form.
- なけれ is the Hypothetical form.
- なかれ is the Imperative form.
- なくない is the Informal Negative construction.
- なかった is the Informal Past construction.
- なくなかった is the Informal Negative Past construction.
- ないです is the Formal construction.
- なくないです is the Formal Negative construction.
- なかったです is the Formal Past construction.
- なくなかったです is the Formal Negative Past construction.
- なくて is the Conjunctive construction.
- なければ is the Conditional construction.
- なかったら is the Provisional construction.
- なかろう is the Volitional construction.
- なさ is the Degree construction.
As you can see, there are many ways to conjugate this adjective. If you wish, I can expand on this answer.