Unfortunately, the plugin doesn't allow for this particular kind of alt text scenario. The plugin will try and get the alt text from the attachment post meta (for images from thumb or attached, so you can edit this in the media library), from the original image tag (for images from first), or from the custom field (for images from customfield).
In the case of customfield, you must also provide the name of the custom field the alt text can be found in
[image from=customfield cfname='name-of-custom-field-with-thumb' cfnamealt='name-of-custom-field-with-alt']
And you can supply backup alt text like this
[image from=thumb alttext='text used if none other is found']