overwrite a template file

I want to modify a plugin that among other things, adds a special form below the backend post editor. I’ve got a post editor on the front end of the website, and I would like to call up specialform.php on this page (frontend.php).

How do I overwrite frontend.php in the plugin so that it adds specialform.php to the bottom?

Note: I’ve never created/edited a plugin before so please paste code or give me a specific example of this.

I’ve looked all over and can’t figure this out so any help is much appreciated!!!