Can I use a list to make a choices for a cascading dropdown for a library form?

I am trying to make a form for a document library.  It is currently empty as it is a test environment to create the form that will later be merged with an active library. This is my first time using Nintex to make a form for a library.  Can I use an existing list for the choices in my dropdown?  I can't figure out how to tie it to the existing category from the library and this needs to be cascading. Any help is much appreciated as there isn't much I can find online about the new responsive forms.