@adx
It's a bit hidden in the docs, but there is a section called "Using your own certificate". Also, based on the language at the top of the same doc, you can simply replace the files "tls_ca.crt" and "tls.pem". I haven't tried, but what I read makes sense to me.
https://docs.pi-hole.net/api/tls/?h=cert#using-your-own-certificate