I do my flightplan with Navigraph and save it as a *.pln file.
It would be nice to load this file like a simbrief file direct in the load window of the plugin.
Why only simbrief?
Thanks
This is not immediately technically possible, you cannot read a file from the filesystem in the environment that is available to us in the plugin.
In the future, you will be able to use the in-game panel to inject a flightplan directly into the avionics. You could also make potential workarounds by spinning up a server on your computer that hosts all files in a “Flightplan folder”, but none of this is straightforward and most of it is hacky or unreliable 
It’s great that this topic exists! Now we can gauge interest in this feature by other users as well.
Kind Regards,
Malte