Subdirectory on Subdomain?

I have a wordpress multisite setup on my parent domain set to create subdomains for child sites. This is working well and has been set up for a couple of years with a few child sites.

One of these subdomains is a MediaWiki site that I would like to move to a subdirectory to supplement one of the existing subdomains (so URL becomes child.parent.co.uk/wikisite) for technical, SEO and UX benefits. The child site needs to retain its current theming and content, so the wiki would become a site within a site (as I understand it).

I can’t figure out how to set up the new subdirectory and move the existing files to it. I’ve edited the wp-config file so that it should be possible but I can’t find a way to create the subdirectory and the WordPress back-end isn’t giving me the option, I assume by design. My hosting provider has been no help.

I haven’t found an alternative to creating a subdirectory that would enable me having a MediaWiki (or better alternative if one exists) setup within the child site, either.

Help! Please and thank you.

 

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