adding the plugin via a form

Greetings,

Unfortunately I do not believe this will be accomplished without some custom code. You may be able to add the shortcode to something like Contact Form 7 or another form editor but I am not sure.

If I understand correctly you should just be able to add the file uploader to a form. When a new file is uploaded it will add a hidden field for each file with name="wp_multi_file_uploader_[NUMBER]". Where [NUMBER] is equal to the file upload count starting at one and the value will be the attachment id. When the form is submitted you can use these details to do what you need with them. The link to the documentation was broken on wp.org I have now fixed it there is a little more information here.

Thanks,
Dan