Mainly the import feature is to restore an already exported backup of the quotes database, however it is possible and seemingly very practical if you already have your quotes in a spreadsheet or such format.
Perhaps as an example to help refer to, you could enter a few quotes into the plugin and perform an export. Then refer to that export files structure as a reference or template. I expect that will work if formatted correctly. Character encoding, if you use copy/paste, can sometimes cause a problem (apostrophes and such). I have some basic error reporting on import that should tell you what line in the file, if there is an error in the formatting. Hope this helps and love to hear how it goes for you. Thanks!