[KevinH]

Yes, use Sigil's Find and Replace in Regex mode.

Have you ever searched using regular expressions before?

The idea is to find all the text between the opening title tag (while capturing the title tag contents) to the end of the opening body tag, then creating a replacement using the full capture with the addition of h1 tag with the captured title value appended.

This is also something you can easily do in a Sigil plugin if you know python, too.