In Local 5.x, if you can edit the sites.json file in:
~/Library/Application\ Support/Local/sites.json
That file holds all the settings for all the sites. You need to find the site you are looking for. You can search for the directory name: site-name, and then after that find the multiSite setting. Something like "multiSite":null or "multiSite":"". Change that setting to "multiSite":"ms-subdomain". That will be a subdomain set up.