Unusual subdomain setup

Suppose I have a wordpress site located at the subdomain https://foo.bar.org. As far as I can tell, the core wordpress multisite subdomain functionality only allows me to set up subdomains with the scheme “https://fiz.foo.bar.org” or “https://ban.foo.bar.org”. How and/or with which plugins would I set this up to have my alternative sites at the same level of subdomain? For instance, I would like https://foo.bar.org and https://fiz.bar.org to be two of the sites in this multi-site.

Setting up https://bar.org as a wordpress multisite is not an option.