I have a well established Thunderbird setup on my desktop and would like to use K9 on my android tablet but would rather not have to manual do ALL the settings for my several email accounts. So what Thunderbird file do I put on a USB stick to get to my android tablet and then how do I import that into k9 already installed but not yet configured?
Tried BlueMail on the android - but don’t like it at all.
Thunderbird and K-9 both have Backup and Restore options in settings and I’m pretty sure 1 can import from the other. So back up your stuff on 1 device, copy that file over to the other and import it there, should work 
Edit: Never mind me, I totally overlooked the “desktop” part of your setup. I probably shouldn’t post right after getting up, my bad 
I’m not sure you can go from desktop to/from mobile at this time.
There was some work on the mobile side to capture QR codes from a beta/nightly build of the Thunderbird desktop app. But I’m not sure if that got officially released.
Otherwise, manual email accounts and passwords. Hopefully it can find the fetching/sending servers for your accounts, else those too.
I installed K-9 on my Android phone; my ThB settings (on laptop) were taken over automatically.
That workes fine, just like it had always done with FairEmail, which I still use on my tablet.