Imports from other sources than LastFm?

A lot of software already supports directly submitting to ListenBrainz. Some is listed at Adding Data - ListenBrainz. A more comprehensive discussion is at List of native ListenBrainz API apps and in development apps

Some tools with audioscrobbler (aka support only can be configured to submit to LB directly using the compatibility layer by configuring the tools to use

As for importing a CSV file directly I am not aware of a ready made utility to do so. But it can be definitely done with some programming and using the ListenBrainz API. There is an explicit “import” for submitting previously recorded listens. At the very minimum this requires you to have a list of listening timestamp, artist name and track title. The release title and the corresponding artist, track and release MusicBrainz IDs are also very helpful.