You can do it without Sigil. Epub is an archival format, like zip. Backup your epub so that you would have a copy in case something goes wrong. Then open it with a program you would normally use to open zip files, and unzip it (the epub file), like you would normally unzip a zip file. (If your program doesn't recognize the epub as zip, change its extension to zip. Unzip. And make sure you have a backup before that.) There will be a bunch of files, look at the html files and find where you want your inserted text to be. Insert it in the html, then zip the whole thing back and add this new zip wile to Calibre as one of your book's formats (I am assuming you have pdf and epub, add the zip, too). Now convert *the zip* to epub. Look at the resulting epub with Calibre's e-reader. You should see your change.
Again, make sure you back up your epub before you do anything with it.