execute Yii2 console command from Yii2 extension with Craft
ok, found out how this works:

it's not sufficient to composer-require the extension
it needs to be activated. for bizley/migration this is done via controllerMap in e.g. app.php (see documentation)
you can now create migrations for existing tables, but they are saved to Craft's vendor director (vendor/craftcms/cms/src/migrations/)
since this is not the right place, I manually created a migration via Craft's console command, copied the code from vendor/craftcms/cms/src/migrations/, removed the automatically created migration and pasted it into the manually created one.