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Did you install the package with composer first? Also that library seems to be discontinued (https://packagist.org/packages/phpoffice/phpexcel) and replaced by https://packagist.org/packages/phpoffice/phpspreadsheet which is the one I use
Once the package is installed via composer, you can include the clases like:
use PhpOffice\PhpSpreadsheet\Spreadsheet;
...
1
You will need to create an admin theme and register it. The solution applies to all common Magento 2 Admin customizations that can be done by just overriding a phtml template. You will just have to copy and place the template to customize at the right location (<theme_dir>/Magento_<MODULE>/templates/<template>.phtml.
Creating the admin ...
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