Is there a way to associate a Hub site into another Hub site?
I'm trying to create a network of sites collected under regional hub sites and in turn associate those regional hubs under a bigger hub site.
This is not supported. Site collections can be associated with hub sites, but you cannot associate hub site to hub site. In general, we would recommend using our community forums for these kinds of questions as this is not really a feature suggestion.
See for example http://aka.ms/sppnp-community.