Updating Lets Encrypt cert in Snap

Can the lets encrypt cert be modified outside the snap and installed in the package? The included nextcloud.https-enable utility worked great but I want to change it so my host responds to a subdomain rather than the second level domain. This looks doable through the lets encrypt command option -expand, but clearly this would have to be done outside snappy.

Is this an application for the custom option?
https://github.com/nextcloud/nextcloud-snap/issues/199