and easily ingest/import the a csv into an external DB, which are not “connected to the Recorder”
But yes, importing csv is another story 
There is this
Recorder - Spook 👻 a scary powerful toolbox for Home Assistant. .
GitHub - klausj1/homeassistant-statistics: This HA integration allows to import and export long term statistics from/to a file like csv or tsv · GitHub .
So who knows when it becomes a Builtin Feature