A plugin that adds a spreadsheet upload to Nested Forms (Gravity Perks). Currently only CSV is supported, you can include a library like PhpSpreadsheet for Excel, Gnumeric or more support.
Install the files as a common Wordpress plugin and activate it. Since there is no admin panel configuration at the moment, you have to touch all files except class-gpnf-session-override.php manually for form field mapping and template modifications. You should be capable reading the database, especially wp_gf_entry_meta table, because you need to know which field ID should contain which value. Everything you have to change in this code is marked.