It is absolutely possible and any custom metadata schema that you create has to applied to a folder to allow the assets within that folder use the metadata schema.
It means if you have defined a custom metadata schema and applied to a folder let's say /content/test then all the assets within will have that metadata schema available for editing.
If your question is about the location where metadata schema is created then yes you are right that it gets created at /conf/global/settings/dam/adminui-extension/metadataschema/default but you can change it for a custom metadata schema. For example if you name your metadata schema as corpa then its location will be /conf/global/settings/dam/adminui-extension/metadataschema/corpa
Please copy the default schema and then modify as a best practice.